Incest does not quality because of the potential for biological/genetic complications in children produced from such a relationship.
Homosexuality cannot be compared to incest, pedophilia or beastiality because it does not deal with the same issues.
The purpose is to 'control urges' that have tangible negative impacts on society. Theft, murder, rape hurt the entities such actions are directed against. Pedophilia exploits the immature and susceptible mind of children, incest can result in the exacerbation of genetic mutations/disorders.
Homosexuality, when engaged in by 2 consenting adults, inflicts no tangible harm on society. I didn't decide to go out and hook up with a guy Friday night because gay marriage was suddenly legal all across the US.
So unless you can show a tangible harm to society from homosexuality, there is no justification to oppose it and ban it.