What if they someday find a "gay gene" (or even just those for various intersex conditions) and cure those?
"Would you like to be heterosexual, just like everyone else?"
(The interesting thing about that is that you can piss off both sides of that debate. What if, in the future, being gay or not was indisputably a choice thanks to medical science? Would those who chose to cure themselves be seen as traitors or...?)
Offering the choice is not the same as shoving it down everyones throat.
Add in that the opposite choice would be true, the technology might allow one to be made gay.
"Would you like to be gay, bisexual or asexual, unlike everyone else?"
People can go to great lengths to modify themselves to not be like everyone else, to stand out.
Bisexuality would double your dating prospects, you can bet some would opt for it if there was a inexpensive treatement.