Wikipedia on causes of homosexuality says there are "various biological causes", and quotes:

The American Academy of Pediatrics stated in Pediatrics in 2004:

Sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences. In recent decades, biologically based theories have been favored by experts....

The American Psychological Association states "there are probably many reasons for a person's sexual orientation and the reasons may be different for different people

We're interested specifically in the heritable component, which seems to be relatively small. There certainly seem to be some specific genetic loci involved, one on the X chromosome and another probably on chromosome 8.

At least one proposed etiology says the mother's immune reaction to the fetus affects its sexual determination, so we should be looking at genetic correlations in mothers of homosexual children as well as in the children themselves.