Wait, who did Cain marry? And that was the second Biblical "marriage"?

Funny that people who fight the most about the sanctity of marriage go the "incest route" and also hold that the Bible is literal and have no issue with Cain having a sister for a wife... or that Isaac had a half-sister for a wife and Jacob with is half-sister wife. So, what isthe definition of Biblical marriage? No, I am not saying that is good. However, here is the thing. God allowed divorce because of the hardness of man's heart so really I could build a case for it. Nevertheless.

Incest damages a person. The psychology of a child is damaged in ways that will affect that child as they grow. While being Gay is something more of a self-realization, incest is a choice made by an adult against a child. Not to also mention that if a child is conceived it has a 42% chance at being born with some serious birth defect. Some argue that just make sure they are sterile... 
Another reason against incest is that it is gross. But morethis. It is linked to mental illness.
http://articles.latimes.com/1992-12-13/news/mn-3971_1_sexual-abuse 

From this article:

"In their study of 75 area women receiving psychotherapy, Dinwiddie and Pribor found the 52 incest victims in that group each, on average, had suffered seven psychiatric illnesses, more than twice the number suffered by the 23 women who hadn't been sexually abused as children."
Mental illness, birth defects, abuse of power in relationships... all lead me to say incest is not good and is different than being born LGBT even if the second biblical "marriage" was Cain and his sister and our biblical patriarchs married their half-sister and no one seems to worry about that at all.

If you think I am a bit overboard here check this site out.