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My understanding of this particular field of research (as it applies to men, not women), is that the relative levels of testosterone/estrogen at critical developmental stages of the embryo/fetus determine a man's sexual preference, and it can be anywhere on a continuum.
Those hormonal levels can be influenced by the genes of both mother and fetus.
They can be also influenced by environment. For example, the more male babies the mother had in the past, the higher the estrogen levels she produces to counteract the androgens produced by the fetus, and the more likely the baby is to grow up to be bisexual or gay. So the more older brothers somebody has, the more likely he is to be bisexual or gay.
So there's a complex mix of genes and environment, but ultimately the decision is made in utero most of the time.
And that applies to men only. I'm not familiar with the corresponding research in women.
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