Do you still believe that R1b L51+ originated in western Europe and spread from there? I don't. I think all the modern genetic evidence shows that it spread from the east. However, up until a few years ago a lot of people believed just that. You can still find people, including people in the field, who do believe that. You can go back just a little bit further and look at the disputes over whether agriculture reached Europe through cultural diffusion or arrived with a new population.
This situation is analogous. Yes, you can find isolated people in the field who still hold homosexuality isn't innate, but the evidence is mounting that this is indeed the case. Notice that I said homosexuality itself, not people who sometimes engage in homosexual sex. For goodness' sakes I sometimes wonder if some of the people writing these things are parents. Or maybe they're the kind of parents where only the nanny takes the kids to the playground. You can see it in some little boys when they're as young as four or five. One of the little boys at our club was like that, and he wasn't the only one I've observed. Are all homosexuals like that? No, they're not, but a good number certainly are. Why do you think even barely pubescent boys are singled out for beatings? It's because they're different.
As for this claim that none of the Greeks and Romans were exclusively homosexual, that's an absurd claim, imo. For one thing, there's absolutely no way that anyone can know that. More importantly, however, the indications from actual writings of the Greeks, for example, is that there were men who did focus exclusively on younger men, and they were condemned or at least mocked for that. The writer of the article obviously never studied the classics.
As for your last question, incest taboos exist for a very good reason. Most people have inculcated them. So, I think it would be extremely rare for anyone as an adult to develop a sexual "love" for their own parent.
In addition, however, I think there is a state interest in ensuring that marriage should not occur between close relatives because of the danger of defective children, and the burden of them eventually falling upon the community as a whole. That's why in the west most jurisdictions forbid marriage between siblings, first cousins, etc.
I've only ever heard of one case similar to what you're suggesting. A woman met her birth father as an adult and they began a sexual relationship. I don't know if it was a hoax or legit, but that's what the parties said. She also said they would never have children. I can't even contemplate such a thing, but if you're asking me if they should be stoned to death or something, no, I wouldn't agree with that.
Now, I've said about all I have to say on the subject.