It is not necessary to believe something is allowed by scripture in order to believe that it should be legal in our society. There are many things that are not allowed in scripture but that are accepted as legal, and rightfully so. Among them:

Divorce for "irreconcilable differences"
The worship of other gods
Gluttony
Lying
Lust
Dishonoring your parents
Drunkenness
Taking God's name in vein
and many others.

If we were to base our legal system on what is and is not allowed by scriptures, we would live in a country where all religions but our own are outlawed, divorce is next to impossible, Buffet dinners and other "all you can eat" offers are outlawed due to their gluttonous nature, Lying is punishable by imprisonment, Lusting after someone else's spouse is a punishable offense, doing something that dishonors your parents is a criminal act, having too many drinks is cause for being locked up, and a person could be imprisoned for uttering the phrase "God damn it!".

Eventually, what you wind up with as a result of using the bible as your basis for laws is a "Christian" version of the Taliban.

Therefore, it is obvious that we must go beyond what the bible says in order to establish laws that are practical, reasonable, and fair to people of all faiths and belief systems.

That being the case, I see no problem with a Christian supporting a fair and just agenda despite the fact that it is not in line with his own interpretation of the bible.