Likely based on some book by a homosexual who spoke about how many partners he's had and how many diseases he's had. IIRC, he claims it is common for gays to have lots of sexual partners in the book.
Probably going to get shit for this, but
>sexes and sexual intercourse have evolved for reproduction
>therefore same sex sexual intercourse goes against this evolution; no offspring is produced
>same with incest, though only sometimes are there miscarriages; generally just retards
>therefore, thought logical, ancient man, homo, zoo and incest are bad and unnatural/evil [with pedo being specifically prepubescent (i.e. not modern conception of "16-18 = okay", though it should be noted that the Catholic Church centuries ago had pushed for similar ages (as opposed to f/m ~14/25 that was common for pagans) and supposedly around ~19)]
And never mind the other aspects about behavior that have been observed (e.g. excessive lying). Also, the occasional unfortunate barren man or woman is the exception, as in normal circumstances they would produce normal offspring, whereas you will never produce children with gays and animals (dunno if chimps have been tried but I'd imagine any offspring would be fucked up or even die on arrival), nor are you going to produce non-deformed or mentally retarded children with incest in nigh all cases (I'm sure there are some where it wasn't such a problem, but 99% nothing good will come).