Could you personally choose to be homosexual if you wanted to?
I believe so, yes. This would be the equivalent of asking, "Could you personally choose to steal something?" The answer would be yes. I highly doubt stealing is something you are born with.
So you should try it for a while, if it's so easy to switch back and fourth. You wouldn't have to actually participate in homosexual sex, just make up your mind that for the next 2 days or so you are going to be attracted to people of the same sex. Actually make a sincere
effort to find people of the opposite sex attractive.
If you are a Christian you should have no problem doing this, since you can always ask for forgiveness after the trial period is over.
If you are able to be seriously attracted to someone of the opposite sex during that time then I submit that there are two possibilities: 1) you are actually homosexual or bisexual and don't know it, but have temporarily removed a strictly mental cultural/religious obstruction to your sexual preferences or 2) that you really can just decide to be homosexual and then turn it off, as you suggest.
It seems to me that the first case is far more likely to be true, since we've seen plenty of failures of homophobic people to cure themselves of their homosexual tendencies. The OP being a perfect case in point.
If you are not able to be attracted to someone of the opposite sex then it isn't something you can choose, either because you can't remove the mental barriers erected by your culture/religion or because you simply aren't homosexual.
Aside from all of this, the fact that homosexuality is such an incredibly powerful temptation to avoid ought to be a concern for any homophobic Christian. If there exists a temptation so incredibly powerful that even people who have invested their entire lives
in opposition to homosexuality succumb to it, why would God torture people like that? It is, for all intents and purposes, so powerful that, for these people, it cannot be avoided. There is, for all practical purposes, no choice for them in this matter. It is so powerful that they will throw away careers, family and friends for it, because the alternative is impossible for them.
That's not a choice.