I'm not disagreeing with you in anyway, men seem to have their own personal ideas that sex is a must have and they must have it (sometimes without the consent of the other party), however it's not only men who have put this idea into our minds. Women seem to assume that men are men (and through out history, due to women being the lesser gender) and women are to perform duties in regards to sex. Similar to the way women were expected to look after the house and children whilst the men went out and earnt money.
This relates now, since a lot of women (not all) are too insecure or don't have enough self-respect to say no to sex when in a relationship with a man who seems to want sex when he wants it, regardless of the woman's feelings. I do understand this frame of mind since I have a similar one, when it comes to me and my boyfriend (though he is extremely supportive and understanding around my needing to feel safe etc in the bedroom) he is a man and I understand that, as a man, he needs certain things from a relationship, just like I do. As a woman, I need to be able to talk and share my feelings as well as (in regards to PTSD) be allowed to be myself and require safety. As a man, he needs (not his only need however) to have a release for his feelings in regards to sex.
Men, in general, have the idea that they are the more important gender, this has gone on since the beginning of time and it wont change any time soon. This idea that they are the superior being makes a group of men (those who rape or take advantage of women) assume they can treat others however they like, because they are in control - in their minds. These women should not be blamed, like you said, it is never the females fault if she is raped because it is up to the man to control their urges.
Now, as you said about clothing and appearance being the cause of females being raped - I get where you come from, women should not be blamed for getting raped because they were wearing a short skirt. However, the male being the male, and apparently unable to control their urges, find women much more sexually attractive when they wear short skirts or low cut tops etc, therefore, making women who wear such things more likely to be raped. Also, throughout history, women who have dressed provocatively have more often than not been known for being a prostitute or having a lot of sex, and so in this modern day, when a women dresses a little less decent than normal, men (instinctively) relate women bearing a lot of skin to sex. And because women in history who were these prostitutes etc, often had little consent in the matter (obviously consenting to sex in some regard, but not what went on after that). And, in a sense, women should then take into consideration that this could be a (maybe rare but real) possibility of being raped or assaulted when walking alone at night or wearing short skirts/low cut tops etc.
I would never say it was the woman's fault for being raped since she did not force the man to forcefully have sex with her. However, if you go out, alone at night, wearing a short skirt, then you must consider all the possibilities - or have a little common sense. So I get where people come from when saying that it's the woman's fault, but it's not their fault - it was what they wore that first caught the man's attention, it was the man's fault for following up on those feelings/desires.
When you commented on the elderly and child rape victims not being seen in the same light - this is because the elderly and the young are both considered slightly helpless. At such a young age, children do not know what sex is, let alone rape and so people treat young children differently due to the fact they don't know what happened, they didn't dress provocatively or lead a man on, they simply stood and let it happen (so to speak) without knowing what was truly going on. Elderly people who are raped are very similar, young men especially tend to intimidate elderly people, due to the stereotypes of gangsters and these rapists and such. So, if an elderly women was approached in her home by a young man who then proceeded to rape her, she would 1) be quite helpless, elderly people physically aren't as strong as a young man and 2) would most likely be afraid out of their minds to do anything else to stop it out of fear of being hurt more, or even killed in a rape gone wrong (similar to a robbery gone wrong, which happens quite often to elderly people - so seeing these stories of elderly people stopping a thief and it all ending badly, would probably be in the very back of their mind producing a fear to do anything).
It's also not uncommon for them to post the hoard of other symptoms the sufferer might have had over a long period of time, but the focus - or seemingly main motivation - for them posting for support is due to the lack of sex issue.
Sex is a big thing most, normal relationships. I can't ask you to imagine it, as I can't, but try to think of a man being in 2 or 3 previous relationships and having sex regularly (both parties willingly) and then meeting a girl with PTSD and sexual issues. He would most likely be confused, a little lost and wanting to know what to do to solve this problem. Instead of assuming all men simply want sex and will go else where if they can't get it, try picture how they feel. It does wonders for learning about how I feel since I can then look at myself from a bird's eye view, so to speak. Anyway, I digress. Like I said, sex is quite a big thing for most men and most relationships - it's crucial in a lot of relationships as it is how both the man and woman bond emotionally and physically (there is a lot of science behind this, go google it). So when a man doesn't get sex, or even if a woman doesn't get sex, they can often feel neglected or alone and as though their partner doesn't love them. As is common with PTSD, showing feelings for someone (such as love or admiration) is quite difficult due to the trust issues, so unless a supporter FULLY understands this, they can often wonder why their sufferer doesn't want to have sex - they then might think it's their fault, that they're not attractive, that the sufferer doesn't love them, that they've done something wrong, and so on. I think in a relationship with someone who has a mental disorder/issue, both parties MUST talk about what each other needs from the relationship. I did this with my boyfriend, I told him I needed security and safety and for things surrounding sex or being intimate to go at my own pace and he told me as long as I showed him I loved him - which involved sex and being intimate, he was happy with how I was. Which was sweet, but it let me know that I must at least try to give him what he wants, when I am comfortable with it.
The men you speak about, who treat women as a possession, first of all, I highly doubt a woman with PTSD would fall in love with a man like that, and also, those men rarely come into a intimate setting with a women (except for if a woman is raped/assaulted), meaning that to everybody else - without PTSD - men like this, are a norm. It's gotten to that point in society where men believing their are superior and own women, is an accepted trait. Obviously, those with PTSD with a sexual related trauma, think differently as their mind has been rewired and so they look at this differently and see it as much much clearer thing in society - obviously there are some people who see this this way without having PTSD or any life events causing them to think differently. I thought differently before my trauma and I think the same now, just heightened.
Also, a supporter might be seeking therapy to help cope with their sufferers symptoms and want therapy to learn how to manage it and cope with it themselves. However, as I have recently learnt, talking about sex and feelings around sex is a very difficult and awkward thing to do. So, speaking about it online, is possibly their only (comfortable) option. Hence the fact their posts seem as though they are focusing on ONLY sex. I can't speak for everyone and I have no doubt there are some men who think sex is the be all and end all of a relationship, but from a lot of the posts I have seen on here and from my own experience with my boyfriend, sex isn't everything to a man who cares and genuinely supports you. (Just a bit of unrelated laughs; I made my boyfriend wait 1 month before I would kiss him and then it was 5 or 6 months before I had sex with him. He was perfectly fine with waiting since by the time I had sex with him, he pretty much knew everything and the fact he waited, showed me that 1) not all men are the same and 2) that he is amazing and one to keep a hold of).
I understand every point you have made and I agree with 99% of them. My advice, try to look at it from both sides. Obviously, rape is inexcusable and men should learn to accept that they can't have everything. But some men on here are genuinely caring and loving partners who are seeking help about the hard to talk about subjects - i.e their desire for sex with a partner suffering from PTSD. Quite the difficult topic if you ask me.
I am very sorry for how long this got, when I feel strongly about something, I tend to babble so I do apologise. I hope some of it is of help/interesting to read at least. ^^