If you're interested in the causes of homosexuality, check out these two links:
http://www.narth.com/docs/pieces.html
How Might Homosexuality Develop?
Putting the Pieces Together
http://www.freerepublic.com/focus/f-news/1420619/posts
Root Causes, Homosexual Consequences
Obviously this group of people are already biased toward a certain set of beliefs. Also, their personal opinions, while they might be interesting, are not a scientific way of determining the true cause.
In the past few years I have seen a lot of threads on FR about this topic. Many of them are put forward by the same group of people who react violently against the idea that there could be any biological connection to homosexuality. One of their strongest core beliefs is that a person is only homosexual if they indulge in homosexual acts. If pressed, they acknowledge that the so called "ex-gays" are not entirely free of sexual feelings toward their own sex.
In my opinion the proper definition should be that a totally heterosexual person does not feel any sexual attraction toward their own sex. Some people do feel these attractions or they have felt them in the past, but they don't want to be considered "gay". But the fact that they do feel these feelings indicates that in some way their brains are different. How these differences arise is a scientific question which should be settled by science and not merely by argument from opinion.
As for the influence of mothers and fathers, it is also true that for many years parenting was blamed when children developed schizophrenia. This theory hurt a lot of people and has been totally debunked.