If you want to change the law to protect the 'sanctity of marriage' then how about this: a couple may only get married after they have been in a relationship for 5 years.

Could be a problem if a couple of mid-30s wannabe breeders meet. They'd want to be dropping sprogs in advance. I bet that wouldn't do much for the sanctity of marriage in the eyes of the Bible Belt. "Fancy those sinners having children out of wedlock! Oh, the humanity!"