@Friday hits the nail on the head. Sexual function (and dysfunction) is so tied to our mental well-being, it's not surprising at all to see sexual interest disappear. My sufferer and I went two years without sex (and prior to that another two-ish years), before finally starting to call it quits. It was partially his work-related injury and physical pain, but mostly his PTSD (which I know now, but spent 7 years thinking he just was rejecting me...boy howdy writing that out makes me realize how badly my boundaries were broken). Of course there are some for whom sex is an outlet and coping mechanism (it is for me for my depression and anxiety...so not getting it and feeling rejected did a number on me).
I actually have suspected he was sexually abused in some way in his formative years, because touch is such an issue with him. Or maybe it's the nature of his complex-PTSD being sourced by physical abuse in general. I'm still not sure, and probably never will be (he's not even entirely sure - it's taken him 7 years to admit that physical contact is an issue). And I try very hard to be all about consent - the only "yes" for me is an enthusiastic "yes." So I certainly did what I could to not trample his own boundaries.
One good thing our couples counseling brought for me was finally hearing that, no, he wasn't rejecting me. It was never about lack of desire or interest or attraction, it was about shame, feeling inadequate, and feeling inexperienced, mixed with some issues of control. Ok, that sounded bad - it SUCKED hearing him have such a low opinion of his physical abilities and having such self-esteem issues. It was good only to know I wasn't being rejected. The reason behind it is shitty and it's another thing that makes me angry at the PTSD monster.
But yeah, it sucks so bad thinking you're being rejected. I've embraced the mantra "It's not me, it's his PTSD. It's not me, it's his PTSD..." etc. It doesn't always help, but some days it does. And it certainly didn't help in the midst of being told "No" literally any time I tried to even remotely get sexy. Kissing and hugging was fine, but anything more...nope.