(Making my own post so that I can write more and I don't hijack someone else's answer).
Well, thus far, no one has come up with anything conclusive to what "causes" someone to be gay. People generally fall into four different camps of thinking: people are "born" gay; enviornmental factors cause someone to be gay; a combination of the former (i.e. perhaps certain things make you more predisposed to being gay, but environment plays a role as well); or that being gay is a choice.
There are studies that suggest that people are born gay. This study is one of them ( link
). What they're unsure of is whether or not it's caused by genetics, or caused by the environment of the womb. Either way, it suggests that a person is born gay (or at least born with a higher predisposition to being gay). Another study found that the more older brothers a boy has, the more likely he is to be gay. The mechanism is unknown, but it's thought to have to do with the mother's body seeing male fetuses as foreign and developing an immune response to them (which would become greater which each subsequent male fetus). link
This is, of course, suggestive that it is related to the environment in utero.
Your view is that all children are a "blank slate" and that homosexuality can be potentially learned due to experiences throughout life. When you say "children", I assume you mean at the time of birth. These studies suggest that there are things going on before birth, while the fetus is still developing. Even if we were to describe a fetus (at the time of conception) as a "blank slate", certainly we know that the environment in-utero can have an effect on the fetus' development (diet, hormones, habits, etc.).
These studies would back up either a) people are born gay; or b) people are born predisposed to being gay (but environment will ultimately determine whether they are or aren't). They do not support the ideas that being gay is a choice, or that being gay is based solely on (post-utero) environment.