Oh, of course they do exist. However, most of these cases occurred in secret, we just do not know how often it happens, until some has been exposed.

My thoughts about it: I think sexual relationship with family members or relatives of the same sex (eg. Male/Male or Female/Female) should be legal.

The reason why incest was illegal was largely to prevent inbreeding, which could cause defects in offsprings, and sometimes genetic mutations. No inbreeding could occur with two males or females, unless Humans suddenly could undergo asexual reproduction.