RE: The question isn’t whether same sex marriage is constitutional but whether it is constitutionally required which of course it most certainly is NOT.
Eventually, the argument will touch the equal-protection clause. That’s what the pro-gay marriage folks are gearing up for.
The equal protection argument is bogus and even if it were valid the courts have other avenues if it is simply a matter of conferring the same benefits to those in otherwise legal civil unions. They could require the separation of benefits from marriage licensure which in my opinion would be a better path concerning equal protection.
The argument for equal protection though is weak. A person chooses whether they will go into a same sex union or not. Many people who are now in same sex unions have previously been legally married so the idea that they can not choose to be legally married as if they are being actively denied marriage is false. Also many who experiment with same sex lifestyles ultimately do leave them and get legally married. Equal protection is not there to protect someone from their self denial through their own free choices. This idea if confirmed would basically make any conceivable “union” that someone decides to call marriage something that Constitutionally federal and state governments would have to affirm which is crazy.
And after that has been settled the LGBT crowd will bring up the subject of "bisexual marriage". The argument will be, "if gay marriage is permissible because 2 people, regardless of gender, are in love they are allowed to be married, shouldn't a bisexual individual be permitted to marry one of each sex?".