cpwill, the 14th amendment IS invoked because there is gender discrimination.
A man can marry a woman. However a WOMAN can't marry a woman. This is discrimination based on gender as its disallowing the woman to do something a member of the opposite sex can do, specifically marrying a woman.
Rewrite the above to "A white person can marry a black person, however a black person can't marry a black person". Would such a law be allowable under the 14th? Unlikely.
The issue here however is that when it comes to the EPC there are three tiers of severity. Race falls into the top tier called "Strict Scrutiny". This requires that the government must show that the challenged classification serves a compelling state interest AND that it is necessary to serve that interest. Discrimination in marriage clearly fails at this standard.
The lowest teir is known as Minimum scrutiny or "rational basis" scrutiny. In this case the government only needs to show that the challenged classification is rationally related to serving a legitimate state interest. This is extremely broad and it is CURRENTLY the teir where "sexual orientation" is found under, which is why using EPC based on it is questionable at this moment as it'd essentially take judges deeming that it deserves to be higher tier for it to work.
But here's the sticky wicket. GENDER falls between those two as a "Middle-Teir Scrutiny" item. In this case the government must show it serves an important state interest and that it is substantially related to serving that interest.
So because Gender is protected at this middle teir the government would need to show that there is an IMPORTANT state interest in allowing men to marry women but not allowing women to marry women and that denying them that ability is substantially related to serving that interest.
The problem with this is there's been few reason given that CLEARLY can be shown as an *important* state interest and that denying this is substantially related to it. In part because there are already NUMEROUS loopholes to just about every single argument why its of "state interest" already in the law, and if you already have numerous loopholes presented its hard to truly argue that its IMPORTANT that you cut off one particular loophole while dozens of others are there.
Tradition? Marriage has changed routinely over the years, from allowing various races to marry, to changes in the average ages being married, etc.
Sanctity? Hard to argue there's a true sanctity to it as multiple marriages and number of divorses are ever rising
Good Family Environment? There are numberous cases of married individuals providing a poor family environment and numerous studies showing that same sex couples can provide a good family environment.
Promote Procreation? Nope, we allow people that are physically unable to reproduce to get married. We allow people who have no desire to reproduce get married. We even allow people who it would be harmful to the child if they were to reproduce, such as the elderly, get married.
While its not an absolute clear cut case that discrimination of marriage based on Gender wouldn't stand up to the middle teir scrutiny, I think its a strong case that it wouldn't and I've yet to see anyone adequetely show a legitimate reason to think otherwise.