What is the current theory of sexual orientations in Biology?
It seems we have more hypothesis than theories.

There is a book called "The Moral Animal", by Robert Wright, where he cites some researches relating order of birth with sexual orientation (of course, not a 100% direct relationship). That's for the proximal cause. For the distal cause (evolutive) it makes sense to me that we are a sort of "eusocial" species, with homosexuality having evolved for the fitness it adds to the family: having lots of offspring, it's benefitial for the family if a few of them don't reproduce, investing instead in other fields of knowledge, like medical herbs, technology, weapons, etc. This may explain why this behavior evolved in first place, and why it's so widespread in virtually every known human culture, and almost all families.

Biologist Alfred Kinsey founded the Kinsey Institute (kinseyinstitute.org) for the study of human sexuality. His theory (that also appears in the movie Kinsey) was that there is a gradient from totally homosexual to totally heterosexual, with many intermediary degrees. Our religious culture tries to "erase" this diversity because of platonic ideals of "perfect male" and "perfect female", but in Amazon I saw that people are much more "plastic" than that, probably because of the lack of power of such religious traditions. Historians also talk about homosexuality as a "usual practice" in pre-Colombian America. And it is said that China have no problem with homosexuality before the arrival of monotheists. In India, homosexuality became a crime only 3 years after the british invasion (so it seems it was very common there, too).