The study of identical twins makes a great case against a genetic basis for homosexuality. The last study I read on the subject showed that in only about 13% of cases when one twins is homosexual is the other twin found to be involved in the lifestyle.
I stand corrected the studies linked here seem to put it at closer to 38%. But the fact still remains that 38% is not 100% as would be expected in genetically identical twins.
The only way that I would see this being a great case is if the twins were not raised in the same home. Still say it is environmental.