If this law passes, there will be a case as Traditional Marriage advocates will sue.
And it will be a tough case to win.
At that point the SCOTUS will Roe v. Wade bans on gay marriage.
Eh... I don’t see it that way. The courts can say “the legislature passed same-sex marriage, and nothing in any constitution forbade them from doing so. They were only exercising their traditional state police power of regulating marriage.”
That is not the same thing as saying “every state must allow gay marriage.” In fact, I think it is much more likely for the Court to strike down DOMA section regarding federal recognition (not the section allowing states to refuse to recognize out-of-state same-sex marriages).
I think SCOTUS will likely uphold state-level marriage bans, precisely to avoid the kind of backlash and division precipitated by Roe.