I'm always astonished when Christians take a firm stand against gay marriage because of what the Torah (or Pentateuch) says. The primary prohibition is found in Leviticus 20 and the prohibition is clearly about anal sex (note "penetration" and "reception" mentioned, although this is not present in the original English texts) rather than homosexuality or gay marriage. So, even were one a firm believer in this law, it would be OK for gay couples to marry just so long as they promise no anal sex.
But, more disturbing is the fact that this law requires both parties (the penetrator and the receiver) to be killed without regard to age, consent, or circumstance. Thus, a strict constructionist Christian (just made that up) would find himself compelled to kill the child who had just been forcibly sodmized by a pedophile. Jewish common law, to be fair, does introduce mitigating circumstances that would presumably deal with this, but I gotta ask: Was God having a really bad day when she thought that one up?