For one, the idea that there is some biological (and that is not the same as "genetic") basis to sexual orientation just seems obvious to me. It's nice to have some proof of it, but it's not surprising.
For two, the idea that it could be due to conditions in the womb is possibly a problem. If the conditions in the womb just increase the chances of adult homosexuality, then there may be no (serious) issue; however, if the idea is that conditions in the womb determine sexuality, then it suggests to me that the anti-homosexual lobby has a perfect avenue to pursue for a homosexuality "cure": genes are hard to manipulate, but conditions in the womb are not. So, change the conditions in the womb and ensure that your kid will grow up "normal".
I'm not for a moment suggesting that scientific research into the origins of sexual orientation should be avoided or refrained from, but I am concerned that the way it's starting to seem like sexuality works -- genes are not determinate one way or the other, womb conditions plus later (non-utero) environmental effects "steer" one's orientation -- is a way that can be mucked around with. And, as with everything we're capable of mucking around with, this raises the inevitable question of whether we should. It's one thing to say that a fully autonomous adult can make decisions about his or her (consenting, adult, etc.) sexual partners without interference, but why not interfere with the development conditions that shape the future sexual orientation of current neonates, infants and children? (I should note that this could cut both ways: parents might want to intervene to ensure that their child is homosexual.)
After all, it's not as if we don't interfere with childrens' developments pretty much constantly -- that's what child-rearing is all about. On the whole, I don't really have a good answer about "why not". It seems like there's no social benefit or detriment one way or the other, which would leave the decision entirely in the hands of the parents.