While I think that the children of homosexual parents may be a little more likely to manifest homosexual behavior, the concept of "learned behavior" means to me more likely being learned from the culture. In other words, in a less sexualized culture, a person who's attracted to people of the same sex may not necessarily express those feelings sexually -- they may just have close emotional relationships with people of the same sex. But in a culture that encourages young people to label themselves sexually at a young age based on their emotional feelings, those feelings may be more likely to find sexual expression.
Maybe, though I'm not aware of any evidence other than anecdotal that would support that theory, and it still does not say that a predisposition, either physiological or psychological isn't in the mix. In other words, the "choice" theory is definitely in the minority. And if the Cheney, Terry, and Keyes children brought up in strict Christian homes with traditional values can turn out to be homosexual, I doubt there's much choice involved.