My husband's highly sexual part is one of his younger parts. We have a LOT of compassion for him, and love him as best we know how, so that he will also FEEL loved. But so far as we can tell, ALL of my husband's parts KNOW about sex. But they all feel about it differently - even his littlest child part "knows" about sex, but in a very sweet innocent and curious and exploring way.
As for "scary looking" .. do you mean she actually presents herself in some way that is "scary" to you? Or is it more just the realization of an adult part that is also sexual that is "scary"? Do your parts have "names" and/or self-identify? For example, my husband's "Femmy" calls himself that as well as being called that "inside the system" of my husband's parts. All of their names, in fact, are (so far): Grumpy, Middle, Femmy, Little and (most recently discovered) High Brow. Their names have more to do with their jobs inside my husband's system, but some folks' parts can have real "people" names, too. In fact, I think that might actually be more common, statistically ...
And my husband's GRUMPY part has come across as very "scary" when it comes to sex. He definitely "likes it rough" ;) .. but my husband always just assumed Grumpy was completely uninterested in sex. So his interest was a surprise! And in reality, Grumpy just doesn't have much patience for EMOTION. But as I have grown in love with ALL of my husband's parts, sex for us has been one aspect of our relationship where ALL of my husband's parts participate in varying ways. Like, he "fast switches" between all parts of himself when we're together. And Middle and Little often worry that Grumpy might hurt me (he doesn't), but I think that's because they SEE him inside as much more "mean" than he really is. Mind you, Grumpy CAN dish out the punishment if he thinks he's being abused, but "sex" isn't in that spirit. Grumpy DOES like to be the dominant "top", but other parts of my husband prefer for ME to be the dominant "top" - so I really try to go with the flow in this area. :)
My husband's Middle and child parts used to be much more freaked out by Grumpy - to use your phrasing. Mostly because they were terrified Grumpy would do things when he was "out" that Middle would usually have to clean up after the fact. And with good reason - Grumpy was a bit of a punisher and would pick fights, etc. And he had kinda a sneaky or sinister glee out of picking the fight, then abruptly going "inside" and Middle would "wake up" having no idea why this other person was pissed off and in his face! But that dynamic hasn't been as true for many years, now. Grumpy is able to be reasoned with, can be reassured that a thing isn't a "threat" when he sees evidence, and he's not just vindictive for vengeance's sake - he's STILL ever the Protector.
As Grumpy and I grew inside OUR own relationship, he was "out" more with the "real world" .. whereas he USED to only be "out" when there was a threat, so his inside view of the real world was very DARK and ugly and mean. Now there are times when he LIKES to be "out" just to have fun in a political debate, for example, or other things that really get his logical mind working - he likes to puzzle things, and he likes to "win" arguments. (LOL) .. and Middle has come to a place where he kinda just "rolls his eyes" at Grumpy's take-charge, sometimes. But lately, I have also noticed that Middle and Grumpy actually get along a lot more inside, too - to the point Grumpy will make a sarcastic comment or observation (which I don't get to hear necessarily, but Middle does) and then Middle will just burst out laughing .. it's literally an inside joke ;) but he will usually share with me later .. "Grumpy just said .. " hahaha! :)
I share all this because I don't want to comment too prematurely on YOUR unique situation without knowing enough. I most definitely don't want to hurt you by making inappropriate interpretations ..
But in the HOPES that perhaps my husband's example might give you some ideas to explore? Or perhaps some questions to ask? Such as of this adult sexual part? And MAYBE she isn't as "scary" as you think .. maybe it's just more her job to be "dominant on top" so to speak? That's not necessarily a bad thing - as long as you are all, internally, working towards cooperation and agreement and communication, etc. :) If there is a POWER struggle, I would really encourage you to keep the communication open, because the more each part of you is OPEN with you (and perhaps with your T?), the more you'll be able to (gradually) organize, inside .. and let each part have his/her "say" in what's going on with all-of-you. ...
BIG HUGS if you accept, dear. This CAN be a scary process .. but I hope you could find some hopeful nuggets in all my rambling! :)
~WU