I would love my child regardless of their sexual orientation. I may have a bit if a harder time if they were bisexual but that would be my hurdle to overcome. I would never tell my mother. She is very religious and although she has a lot of "sympathy" for gays and lesbians she gets upset that they can't change. She knows they can't change but I don't even want the idea of sin and hell being transferred to my children in regards to this. Too many gay people have suffered mental illness and self loathing or even suicide for being socially and spiritually ostracised.
In regards to incest, homosexuality is about finding a particular sex attractive.
If you found your child attractive or indeed children attractive you would be mentally unstable and exploiting minors or abusing if you wanted to carry out your desires.
If you found your adult child atttractive there would still be that issue of it being ethically wrong as you would be in a position of power. Like a doctor and a patient.
If two adults who were siblings found each other attractive then 1. It's not the same thing as finding a whole sex attractive. You can't say finding "sisters" attractive is the same thing as finding "men" attractive. In regards to biology. Having children with a sibling is medically dangerous. You can't have children with same sex couples and hence there is no danger to their offspring. Hence it's not the same thing. You can try and say it is but it isn't. Two men can happily have sex for seven months non stop with no contraceptive and ther will be no child that may have a whole list of congenital diseases. Not the same thing AT ALL. I realise it makes many religious people happy to think that it is but the reality is it isn't.
And lastly, the basic reality is, if it's not hurting another being, what consenting adults do is their business. You can think it's wrong and you can have an opinion (I think people who watch x factor are wrong)
but it's really none of your business. Even if a brother and sister both had the snip and we unable to have children and were getting it on, it's none of your concern.
BUT an adult exposing a child to self loathing, mental illness and lifetime of depression and mental instability because they are constantly told that their natural and uncontrollable attraction to the same sex is evil and unnatural and a CHOICE is actually other people's business as it is damaging not just an adult for a lifetime but a child who should be protected from such things.
People have been gay in all cultures and in all times and we are learning in many species. If being gay is a choice then what age and time did you personally CHOOSE to be straight? Have you ever rejected your urges and hence choose NOT to be gay. I know I didn't choose to be heterosexual. The inclination took hold around 12 and is still with me.
Love and compassion. Live and let live.