Well, to be fair, there is no evidence that homosexuality arises from birth. The best research we have is that there is correlation with factors at birth, but not conclusively proving it's caused at birth. There are many twin concordance studies where scientists looked at the sexual orientation of identical twins who were either raised together or separated at birth and raised apart. If it was something that arises from birth (e.g. gay genes), you'd expect to see almost 100% of twins separated at birth to either become matching homosexual or heterosexual. Instead it's only around 30% of identical twins where one is homosexual and the other turned out to be too.

What we have in science is a "nature-vs-nuture" debate. If homosexuality was exclusively by the way someone was raised, then you'd expect to see a very low rate of concordance in those separated twins (i.e. the twins raised in different homes by different people would have an expected homosexuality rate around the population average, rather than the 30% seen in the studies). There's other ambiguous evidence, like MRI scans that show structural differences in the brains of men and women, and homosexual men have brains that match the shape of women in certain areas (identified as the Sexually Dimorphic Nucleus, SDN ). Whether this is a pre-existing condition that causes homosexuality, or takes shape in the brain in response to homosexuality, is unclear in today's neuroscience research. The best answer I can say is that both nature and nurture are critical factors it seems.

It's not not something that presents at birth; for example 30% of gay couples with children in the US have their children from a previous straight marriage. I'd argue people arrive at homosexuality in different ways; it's through classical and operant conditioning that we develop our likes and dislikes, and our desires are shaped. The idea that homosexuality is present from birth would mean there are gay babies, which nobody takes seriously (even New York Times magazine said that it was an uncomfortable thought that even gay advocates didn't want to venture into).