This made me really sad. :( You didn't need him anyway, he was a jerk. You deserve better than a jackass. Hope it all goes well for you. Best of luck op.

Maybe they went as friends my sister went with her gay friend Sergio to prom

Okay, as much as this sucks, I want to ask a few things. Is he out? If so, to who? Is it well known that he's out? What are the views on homosexuality in your area? Did he have reason to feel it was unsafe to go with a same sex partner? Are they going as actual dates, or does the girl know the date is unromantic? Not saying you shouldn't sit down and have a very serious talk with him, because you should, but because of how people can be about 'nonstandard' relationships, this unfortunately does require a bit more digging. Unless it is a romantic date, in which case, drop him like a ton of bricks.