I think the Equal Protection Clause is a viable route to go. All it requires in this case is a simple question, "Is marriage a legal institution?" If the answer is "yes", then same-sex marriage wins. The family argument will also be used but it will have to go through equal protection as well. The Iowa State Supreme Court used that route, they stated that homosexual couples had a right to raise their children in a legitimate family just the same as heterosexuals. That is still an equal protection issue.
Even Scalia was concerned that Equal Protection would cause same-sex marriage to pass.
"This reasoning [O'Connor's equal protection argument] leaves on pretty shaky grounds state laws limiting marriage to opposite-sex couples."
LAWRENCE V. TEXAS