What was so insulting about sexuality?

Whilst watching Zoë playing Quake Live, I was reminded by the insults flying that aparently no truly cutting insult is complete without questioning the insulted person’s sexuality.

It left me wondering, why are people so tetchy about their sexuality or perceived sexuality? Is it just straight people who get tetchy? Would it be insulting to say to a gay person “you heterosexual, you!”?

I’m bisexual, so I guess it’s just impossible for the inhabitants of parents’ basements the world over to insult me in the only ways they appear to know how.