If it makes you uncomfortable, don't do it. Your boyfriend has no right to pressure you to do something sexual that you don't want to do either.
That said, wearing a strapon doesn't make you a man any more than a man wearing a dress makes him a woman. The kind of sex that two partners have also has nothing to do with the gender or orientations of the people doing them. Plenty of straight cis women wear strapons in bed for their male partners; it's called pegging, and for most women, it's just another kind of sex toy that's no different than a vibrator or dildo that they might use on themselves. The point of using it is, simply enough, to give pleasure and to feel pleasure from giving it. Your boyfriend might really like how anal penetration feels or makes him feel emotionally. It does not mean he is gay or that he is pretending you are a man.
