Darren Main writes (paraphrased):

...if we are going to go by what the bible says about marriage, then take a look at what the Constitutional Amendment about marriage should look like based on biblical principles:

Marriage in the United States shall consist of a union between one man and one or more women. (Gen 29:17-28; II Sam 3:2-5)

A marriage shall be considered valid only if the wife is a virgin. If the wife is not a virgin, she shall be executed. (Deut 22:13-21)

Marriage of a believer and a non-believer shall be forbidden. (Gen 24:3; Num 25:1-9; Ezra 9:12; Neh 10:30)

Marriage shall not impede a man's right to take concubines in addition to his wife or wives. (II Sam 5:13; I Kings 11:3; II Chron 11:21)

Since marriage is for life, no law may allow divorce. (Deut 22:19; Mark 10:9)

If a married man dies without children, his brother shall marry the widow. (Gen. 38:6-10; Deut 25:5-10)

The common response to this sort of thing is that Jesus set up a new set of rules. But Jesus never mentions gays so the only biblical basis for the discrimination of them is in the Old Testament.

And if the rule is to fall back on Old Testament rules where not contradicted by Jesus, then we must use the Old Testament for marriage rules, as I don't recall Jesus saying much about marriage.