And prenups can be challenged. But even without the challenge, there are still things that people are required to be responsible for for their spouse, such as decisions to be made concerning medical issues and death of their spouse. A prenup only changes compensation of money/property existing prior to the marriage to the spouse who didn't own that stuff in the first place.
Levels of Scrutiny Under the Equal Protection Clause
Equal Protection Clause - Wikipedia, the free encyclopedia
The level of scrutiny and how state interests apply are very important to this argument. Ignoring that sex discrimination is allowed, but only when a proven relationship between an important state interest can be proven to be met by that discrimination. There is no important state interest being met by restricting couples from marrying on the basis of their sex alone.