Sadly, the best way to "prove" you're straight is to start dating someone of the opposite sex. And then have frequent PDA in front of the people who doubt you.
A close second is to have a "boyfriend" (or girlfriend) who conveniently lives at least two states away, and who never visits, but calls you occasionally. You just have to pretend to talk on the phone once in a while, and buy yourself cutesy presents for your birthday so you can gush to everyone about how thoughtful your boyfriend/girlfriend is!
I agree that people should NEVER talk about someone's orientation unless the person in question brought it up *during that conversation* (no fair hearkening back to six months ago) and wants to talk about it. There are g a y people "in the closet" or who don't want to share that bit of their lives with particular other acquaintances, there are straight people who just don't want to hear everyone's opinion on the matter or who don't want other people to treat them differently just because they've proven themselves straight, and there are people in between who aren't ready to take a stand one way or the other. Just about the only time talking it's appropriate to bring up orientation, IMHO, is once you're dating someone and before you decide to actually have sex - it's fair to ask a bit about their background (and previous partners of either gender) before making that decision.
"So Miss Slartibart, *OCCUPATION* - is that like if you're straight or something?" (teenage library patron asked me this today while filling out a MySpace profile)