(NSFW) Why is male prostate stimulation stigmatized as an exclusively homosexual act?

This is something I’ve always sort of questioned and not really understood. Maybe a lot of it has to do with American culture and other areas view it differently, but I honestly have no clue.

Why is it that if a man enjoys prostate stimulation it automagicially means that they must be homosexual or at the least, bisexual, even if they find no sexual attraction to the same sex.

The whole logic of it seems asinine to me honestly, it’s totally ok to grab your own dick to masturbate and still be straight, but if you explore other parts of your body, you’re somehow gay, even if it’s a woman that’s doing these acts to you.

Is this nothing more than just a case of people not being comfortable with their own sexuality?