Julie,
Last I heard, the sperm don't check for a marriage license before they start heading towards an egg. "Propagation" and "sanctity" are two different things. But the only people who marry "just for sex" are people who think they have to wait until marriage to <I>have</I> sex.
Not that this has anything to do with anything, but I have a cat named Duke, who I often call "Pooka."
The Lovings were an interracial couple who lived in Virginia in the 60's, when "antimiscegenation" laws were still on the books in that state. They went out-of-state to marry, but when they returned they were sentenced to a year in prison for marrying outside of their race. The case went all the way to the Supreme Court, where the decision that I've quoted elsewhere was made: That marriage is a fundamental right, and that making it illegal on the basis of race was a violation of the 14th Amendment.