An outstanding and very long article on sexual orientation from the Boston Globe. It comprehensively covers all of the prevailing theories on the table about what causes homosexuality or bisexuality.
Most experts who have studied the issue conclude that 1) there is a strong biological component, 2) whatever the causes, the orientation is fixed at a very young age (before five), 3) the orientation is thus, not a choice, but rather a fixed part of one's being.
The identical twins aspect is fascinating. On the one hand, we know sexual orientation is not genetic like eye-color or hair-color because those traits are more are less controlled by a single gene (I'm not a biologist, so feel free to correct me) and are thus possessed identically by both twins. Yet, if one twin is gay, the other identical twin may not necessarily be so. The concordance rate is 50%, which suggests a strong genetic component, but also something else.
The traditional developmental theory, still posited by such groups as NARTH has long been discredited. Supporters of this theory, such as James Dobson, jump on the identical twin discrepancy to attempt to demonstrate that homosexuality is not genetic (i.e., if it were there would be 100% concordance rates like eye-color or hair-color). But as the article demonstrates, when you examine one identical twin who is gay and the other who is straight, there is no apparent evidence of such developmental differences as being the cause of the sexual orientation difference. In fact, not only are the genes the same, but so too is the developmental environment.
The article examines two identical twin boys, right now the age of seven, where one is typical masculine boy, and the other exhibits signs of gender non-conformity, which is a very high predictor for homosexuality (in other words, there is roughly a 75% chance that the boy who likes to play with dolls will turn out to be gay).
As identical twins, Patrick and Thomas began as genetic clones. From the moment they came out of their mother's womb, their environment was about as close to identical as possible - being fed, changed, and plopped into their car seats the same way, having similar relationships with the same nurturing father and mother. Yet before either boy could talk, one showed highly feminine traits while the other appeared to be "all boy," as the moms at the playgrounds say with apologetic shrugs.
"That my sons were different the second they were born, there is no question about it," says the twins' mother.
So what happened between their identical genetic starting point and their births? They spent nine months in utero. In the hunt for what causes people to be gay or straight, that's now the most interesting and potentially enlightening frontier.
In other words, we have two identical genetic entities, and simply by the roll of the dice, one turns out gay, one straight. It's likely that something in utero turned on the "switch" for one, but not the other.