Type 0 blood is recessive. You must have two 0 genes to have 0 blood. Your parents can have A type blood -- dominate -- and you can have 0 only if they have an A/0 gene combination.
Now if everyone with type 0 blood could not reproduce then the percentage of people with type 0 blood would be drastically reduced until it was pretty much eliminated because the only reproducers in society could -- at best -- provide a 25 percent chance of having a baby with type 0 blood.
My point is...even with your theory gayness would eventually be non-existent if it were strictly genetic. Perhaps another analogy would have been hair color with a light and dark potential -- but I doubt sexuality is a hetero/homo potential type case. Still, without being able to reproduce, time would not be their friend.
Now if you argue that it is a genetic abnormality or anomaly your theory might work. Rare diseases and disorders fit into that category. But we do not call them normal and desirable. I doubt anyone would be comfortable putting gayness in that category.
It has been awhile since I have studied genetics but that's the way I remember it.
Besides, then what about rape or other undesirable sexual behaviors. Are they genetic? At what point is a person just responsible for making their own behavioral choices?
The eternal socialist-adolescents of the Baby Boom generation don't ever want to be held personally responsible for their actions. Got to be "someone else's" fault or something they can't control.
The human condition for most of human existance has been tribal. Wolves have packs, sheep have flocks, cattle herd, and people lived in tribes.
Now visualize an advantage to family groups where, say, a couple of guys never "mate" (with women.) They are going to be somebodies brothers and uncles. They bring in game from hunting, or make arrowheads, whatever. The thing is, their "income" is given to the family units of their brothers, sisters, nephews and nieces. Not their own.
They don't have to reproduce themselves. For recessive genes, the genes will be passed on by their brothers and sisters. So - there is a selective advantage to having a small (say 2% or 3%) of the males to be oriented to not mate with females. The advantage goes directly to their immediate extended family and then to the tribe.
They produce more resources than they consume! This can be significant when you consider how close to the edge of survival exist most tribes. A tough winter, a mammoth stepping on a couple hunters, etc. Having a couple contributors who don't have continuously pregnant "wives" and a pack of kids who would otherwise get first dibs on their provinder...that can make the difference on a tribe surviving or not.
This hypothesis explains why there may be a genetic advantage to having a small percentage of males oriented towards homosexuality. (There may be an equal number of females so oriented, but in a tribal society, unless they become shaman/witches/herb girls and so have enough value to be left alone, they get "mated" whether they will or not.) Note this works only for tribes or packs, but not herds (where a dominant male will mate with most females.)
Got it? So there is a hypothesis that explains why there could be a genetic predisposition for a small amount of homosexuality among humans. (Doesn't prove it one way or another, of course.)