I know three sets of identical twins. They are all straight.
Identical twins are clones of each other (they have the same DNA), so by having one gay and one straight twin, it shows that homosexuality is not necessarily coded for in an individuals genes, but possibly gained from experience. (I'm not a scientist so that might be wrong)
Well, if the statement that identical twins share the same sexuality 50% of the time is a true one, that means that sexuality is likely to be coded for at least in part by genes.
Because if it had nothing to do with genes, you'd expect them to have no greater percent chance of having the same sexuality as regular siblings, but 50% is larger than what it is for regular siblings.
2. Identical twins do not exactly have the same genetic state. While their genes may be identical in a dominant/recessive sense, the interactions with the physical environment on the quantum and post-quantum level vary. For example, if one identical twin develops cancer (which has mainly been diagnosed as a genetic disease), then the other must develop cancer simultaneously. Yet I've not seen or heard of such an occurrence. Statistics looking at twins dying within years of each other actually support this line of reasoning, although we'd have to examine each instance of this to eliminate any other non-genetic factors.
I'm sure there are others I haven't noticed, and I don't claim to be a physicist or a biologist. I'll see if I can find others.
Actually, identical twins do have exactly the same genetic code as one another. That's what makes them identical twins.
It's completely true that environmental effects may influence gene expression and that may lead to a different sexuality state. It's also true, and probably likely, that there may be factors completely unrelated to genetics that influence sexuality.
That said, I don't agree with the statement you made about cancer. Say you have a pair of women who are identical twins and one of them ends up having
hereditary breast cancer. There is basically a 100% chance the other twin will get breast cancer. Perhaps not at the same time, but she certainly will. (Just keep in mind, I'm talking about hereditary, BCR-ABL breast cancer, not spontaneous breast cancer here.)
Anyway, just throwing that out there. Interesting topic, though. I'd predict that there's a strong possibility that actual concordance is higher than 50% in identical twins when it comes to sexuality, but that societal influence may pressure at least one of them into not coming out of the closet or denying his or her own sexuality.