There's no hard and fast litmus test for being gay or straight but this question gets ballpark.
When Vestigial says the idea makes him want to throw up, I think thats a normal straight guy answer. Many women would think the opposite; some of us gay guys really want the stuff badly; wouldnt it be odd for a straight guy to actually think yum, yum, sure would like to get a taste of that dudes cum?
That's what I mean by it being a litmus test. It's not like people in this Zip Code want and those over there don't; or there's an age difference, or socio-economic difference - the gay brother wants to swallow some of the stuff, the straight brother thinks he's going to puke.
The OP post leaves me assuming this guy is bi-sexual.
Its like the smell of a man or woman. I knew the aroma of men and women, dont we all? Im gay. The aroma of a man makes me horny; the aroma of a woman, whatever!
Its like how we check out guys or gals. If youre straight, do you check out guys, barely, and not the uncontrollable way you check out women? I assume not.
Maybe if you are genuinely bi-sexual the sex triggers arent gender but something else maybe you respond to the aroma of men and women, check out people of either gender; maybe the idea of tasting another guys sperm is something you would consider.