Finding the act of gay sex relevant to any other discussion of homosexuality makes you prejudiced, because you don't need to know about or endorse a straight couple's bedroom behaviour to see them as normal and/or legitimate, so the same ought logically hold for an LBGT relationship. Like, there's pretty unlikely to be some scenario where you're held hostage and forced against your will to watch sex that repulses you, so typically when this conversation comes up it's a straight person unnecessarily sexualizing homosexuality while failing to do the same thing for heterosexuality.
I know in this hypothetical, the person is held as being "not prejudiced in any other way", but there are many subtle ways in which people are prejudiced, not just "the wages of sin are death" type grandiose statements. If your standards for straight couples don't hold for gay couples as far as their dignity and the fullness of their love and humanity are concerned, you are prejudiced. If your standards for straight people don't hold for gay people as far as their dignity and their humanity are concerned, you are prejudiced.
I agree with this post.
But for the few mentions of being repulsed by two guys kissing: that's some deep issues, and if you're repulsed by that you are very prejudiced.