Forbidden Love

Just watched The Sorcerer and the White Snake. It's based off a chinese legend 白蛇传, which is about a thousand year old white snake that fall in love with a human, and how she was captured and sealed off by a monk because love between a youkai and a human is forbidden.
This plot is somewhat similar to the disney film Little Mermaid, except that it's happy ending for Little Mermaid's case. Once again it touches upon the topic of the supposed 'forbidden' love.
Makes me wonder why is there still forbidden love in the societies of today. For example, love between siblings, love between the same gender.
The most logical explanation I can think of, other than 'it's disgusting / weird / not normal' which is not at all a logical explanation, is that it is not beneficial to the society as a whole in terms of evolution. I'm ignoring religious reasons btw.
Firstly, offspring produced by people who are genetically related (at most by a few generations) have a higher chance of suffering from genetic diseases or disabilities, so it is harmful to the society as there will be more people who needs other people's care. So maybe that's one of the reasons why societies are so against things like incest.
Secondly, for love between the same gender, it's probably because there can't be any offspring so it is a waste of gene pools. It's like if there's a person who is very talented at certain areas, but does not (or refuses to) have any child, it's kind of like a waste of his genes because they can't be passed down.
So it seems to me as though ultimately society chooses what a person wants and can do. The only reason why homosexuality seems more accepted than incest is because no child is better than mentally retarded child?
Maybe there are other logical reasons which I never find yet, because I don't really believe that 'it's disgusting / weird' can be considered as a good reason, because it's up to a person's taste. And one should not impose his own taste onto others right? Although it is true that most people use that as their personal reason for denouncing such behaviours, because others should not impose their personal opinion onto them as well. Ok now I'm confused >.<
Well it's probably cus I'm rather open to influence so I've seen all sorts of things already.
mmm maybe when man tasted the forbidden fruit, there's a forbidden forbidden fruit somewhere that makes him experience forbidden love. oh wow.