Preamble: I believe that good parents stay married. There are few exceptions (physical abuse, sexual abuse, addiction).
The state of California has,as of 1/1/2004, added qualifications a person must satisfy before earning a license to drive. The previous modifications have lowered the teen death rate by some enormous proportion (too lazy to find the exact citations).
But there are NO education, practice, or testing required before acquiring a marriage license.
It may be that making divorce more difficult would lower the marriage rate while not changing (or increasing) the number of babies born to unmarried (or worse yet, unconnected) couples.
I don't have an answer, other than saying banning same-sex marriages ain't even in the ball park.