You
forget that being "born that way" is not
necessarily the same as being
genetic. The
genes are like a set of
instructions - if the instructions are
flawed then you won't get a correct
result (and not to suggest that homosexuality is a
flaw or anything like that of course), but correct instructions do not
guarantee that the result will match those instructions.
Pregnancy is like any other natural process, full of imperfections, variables, random occurences. The path from conception to birth is a long one, with plenty of opportunities for things to occur to shape the life to come of the person. Hormones are often present to guide the development, and more and more people are believing in the theory that the brain is just as receptive to them as the body is, and that it may be more fragile in when it expects those hormoes as to how it develops. (Abnormal timing of hormone releases, and unusual amounts (possibly due to stress) are one of the big theories around transgenderism, with some people thinking sexual orientation can also be affected in the same manner),
Regardless, it doesn't matter that much where it came from, since it's how it is, and it sure can't be changed (no matter what some people try to believe).
Oh yeah, biological does not mean genetic.