I think it's similar to how pro-choice people argue about abortions in cases of rape, even if they think abortion is OK in cases where it was an "accident".
They do this because pro-lifers usually blame the woman for having sex, etc.
Anti-gay people kind of do the same. Like "Want to get married and have equal rights? Don't want to get bullied? Well that's your fault because you chose to be gay. It's your consequence" is similar to "Can't afford a child? Can't deal with the emotional strain of adoption or the financial issues? Well you should have kept your legs closed!".
If it were proven that being gay isn't a choice, then fundies would have to find a new argument. Just like how their argument against abortion doesn't work in cases of rape at all, even though the argument was shit in the first place.