Next, they would need to prove that the state has an interest in denying same sex couples the right to marriage on grounds of procreation but also has an interest in providing opposite sex couples who are infertile, too elderly to conceive, too ill to have children, or who simply choose not to have children the right to marry on those grounds.
Also, they need to demonstrate that interest is greater than the 80 findings of fact that Walker laid down that same sex marriage would be beneficial to the state of California.
Finally, they would need to demonstrate that California did pass the appropriate tier of scrutiny necessary for restricting a right along the lines of sex in order to demonstrate that they did not violate Due Process. You might want to look up the tiers of scrutiny if you don't understand the concept, which appears you might not since you are simply stationg that they did not violate Due Process, without actually providing any reasoning as to how they didn't.
I wish both you and them luck in that quest.