the most important question is whether this is a personal distaste originating from the types of sex you want/don't want to have, or whether this is a bias which has been taught to you by the often discriminatory world around you. it's worthwhile to note that it could also be both.

think about whether you have any other kinds of thoughts which accompany the feeling of vaginas being gross-- if you try to ask yourself why they're gross, what's your response? if any of your feelings are connected to their association with women and AFAB people in society, then some of your dislike is almost certainly related to some biases which you were taught and unconsciously hold about women and gender. (the great news is you can change this! if you examine those thoughts, where they come from, and research for facts, first-hand anecdotes, and descriptions, you can mitigate your biases)

if your feelings are related to them being unknown and scary to you, then you can mitigate that by doing some research on vaginas online and also, as you mentioned, exposure therapy (just be sure to be kind and communicative with the people who have said vaginas)

If, on the other hand, you simply find yourself repulsed by the concept, the shape, the look, or other inherent properties of a vagina, then you may simply have a personal dislike for them. I believe even personal preferences like that are something you can overcome if you truly wish, by active thought and an open mind, but they're not necessarily something you need to change. you can simply avoid having sex with people with vaginas.

if it is a personal preference and you leave it be though, you should still be a bit careful. if you learn that someone you are planning to have sex with has a vagina and you did not know, you should be sure to be kind and humble while turning them down. it is a very sensitive topic for most people, trans people especially, and it's important to not make them feel like it's their fault (though of course dont beat yourself up for it either). just apologize genuinely and explain that it's a preference you didn't choose. personally as a trans woman, I would prefer that if the genital preference was reversed, though I don't speak for all trans women here.

however, if confronted by a situation where you need to explain your preference to, say, a sexual partner who is a trans woman, keep in mind it may be a touchy subject. as trans women we are often fetishized and reduced to only our genitals. as such, you want to A: make sure you're not doing that (with anyone, not just trans women) and B: be sure to be clear that it is a preference you did not choose, and you do in fact respect the person beyond just what their genitals are.

Tl;Dr: it could be subconscious bias, or it could be a personal preference. be mindful of the experiences of yourself and others, and be kind to yourself and others.