I was born heterosexual, but there is nothing about my heterosexuality that requires that I perform heterosexual sex acts at any time during my lifetime. I can either engage in heterosexual behavior or not. And the same applies to homosexuals.
So, even if a person has no choice about whether or not to be gay (I must say that is certainly true in my case, and I am inclined to think that it is true in the case of at least some homosexuals), it is possible to freely choose or not choose to engage in homosexual behavior.
While sexual orientation is many cases not a choice, it has to be a choice sometimes, otherwise there would be no bisexuals.