It's all of these things: genetic, epigenetic, hormonal and cultural. All contribute to sexual orientation.
Hormones can contribute strongly, but mostly in the first trimester of development. Pregnant sheep dosed with oestrogens, for example, will reliably produce homosexual offspring. Hormones also induce epigenetic changes that can be passed down for several generations. High stress scenarios: war, famine, &c, will produce a higher ratio of gays born, as well as a lot of other medical effects.
There are cultural factors, as well. In cultures that don't discourage homosexuality people tend to develop wider orientations. In some cultures male homosexuality or bisexuality is common and accepted as normal.
See Greek and Melanesian models:
http://www.mygenes.co.nz/PDFs/Ch6.pdf