There are no 'qualifications' for ANYONE of opposite gender in this country to marry, except age. So there's an obvious discrimination right there based on gender. Felons in jail that beat their previous wives to death can still marry any woman over 18 and younger if she has her parents' permission in some states.
Edit: SOrry, I misread what I bolded. I thought it referred to opposite genders. Because it doesnt make sense to me as written. Because until recently...and still not in every state...males could not/cannot marry males for ANY reason. Because no reasons are required for opposite gender couples to marry at all.