The best argument is: that's the way we do it in the US. We've never waited for complete agreement among the states before acting on the Federal level, regardless of whether it is a Constitutional Amendment or a SCOTUS decision.

This article shows the number of states that have reached consensus on Women's Suffrage, Interracial Marriage, Prohibition and a few other issues. Same-sex marriage has actually had more consensus than many of those other issues did before SCOTUS jumped in.

http://www.bloomberg.com/graphics/2015-pace-of-social-change...