I think it is perfectly possible for a baby to be born with a gene(s) that causes them to favor the same sex when they mature. Of course babies won't "act gay" just as a heterosexual baby won't start making out with babies of the opposite sex; saying that a baby is born gay just means that they have the gene which will activate at puberty. I think that there may be some people that choose to become gay (or more to the point, to have gay sex), but for the most part it is just something that is intrinsic in them. I've even heard homosexuals lament that they are the way they are; they'd say that if given the choice they'd choose to be heterosexual. Society is set up more for heterosexuals so it is easier to blend in as a heterosexual; there is less stigma being heterosexual; and since the majority of the population is heterosexual, it is easier to find a partner as a straight person.
Even if your belief system states that homosexual sex is a sin, this doesn't negate the fact that some people might be born gay. Even though murder is a sin, a person born paranoid schizophrenic may have the tendency to want to kill other people. Likewise a medically diagnosed depressed person may want to commit suicide which again is a sin according to Christian faith systems. An overweight person may have a thyroid problem even though gluttony is a sin. Etc., etc. At the very least you may want to describe the act as a sin and that a person has a choice whether to engage in that activity. If that were the case, then instead of trying to persecute people for being gay, you may want to try helping them.
BTW, I wish that topics that are this divisive (esp. when the OP seems to be trolling a bit by almost seemingly to be purposefully obtuse to think that a baby will express their sexual preference before there hormones have kicked in to assert late-acting genes) would be posted in the Debate and Discussion section.