There is no valid/rational argument against gay marriage.
Of course there are!
- P1. God condemns homosexuality.
P2. God sets my moral constitution as a believer.
C1. Homosexuality is wrong.
P3. Marriage is in accordance to God's morals.
C2. Homosexual marriage is wrong.
Logically valid, but the soundness of the premises, and therefore the conclusions, can be questioned and criticized.
For example, the argument presupposes God and it also presupposes that marriage is a matter of the Church. The veracity of these presuppositions is questionable.
With re: to incest, it has been suggested that greater degrees of genetic variance stops transmission of defective alleles. However, there is negligible reasoning really to prohibit incest purely on these grounds; it is prohibited because of the potential of sexual grooming and exploitation in the familial situation. Furthermore, in some jurisdictions, it is deemed incestuous to have relations between say a step-father and a step-daughter because an "incestuous" relation is qualified by the social proximity of the couple and not the genetic. It has everything to do with consent and reduction of exploitation of the minor.
The argument of bestiality is absurd. An animal cannot consent or have the cognizance to enter a formal and explicit relationship. The comparison of gay marriage to zoophilia is just some droll hyperbole employed by people who cannot form a coherent argument and wish to cheapen the oppositions arguments with tasteless analogies.
Polygamy is another issue altogether. About the most biologically sound argument; there have been studies done to suggest that the penises of homo sapiens are shaped the way they are so as to scrape out the semen of rival males. Also, it has been shown that men have higher levels of seminal virility in their ejaculates when watching pornography involving more than one male. Men seem to be naturally competitive. I am not sure how well that bodes with our cerebral ethical codes though.
