Only time will tell but, based upon my observations of todays teens and young adults, 90% of them view homosexuality as being normal and acceptable. Within a couple of generations they will be the majority. That majority coupled with homosexuals framing the debate of non-traditional marriage as a civil rights issue will provide strong support for the repeal of any Constitutional Amendment concerning marriage.
Secondly, the overwhelming majority of young folks are woefully ill informed no matter the issue. It is high time that this issue is openly debated and all possible ramifications to liberalizing, read redefining, marriage be placed on the table for one and all to ponder upon.
Thirdly, young people, as a rule, don't vote. Whether that is good or bad is up to the individual's to decide but it is none the less true.