Its because we all want what we can't have. Once a girl sees your into another girl or vice versa, you become 10x more attractive. I hate to admit it but it happens to me. I'll think a girl is just "alright", but then I find out she hooked up with someone or has a boyfriend, I think "omg, that girl is datable and desired" and she becomes hot.
I agree that for the most part, people want what they can't have, especially girls. You're probably on your own though with what makes girls desirable. :lol
Its like bitches just know when you're with someone and all just come on to you. Next time this happens I'm ****ing everyone.