That is the way I see it too.
American Traditional moral values (which are dying unfortunately) that we grew up with would be married hetrosexual opposite sex parents of children, going to Church on Sunday with others alike.
Liberal (modern) moral values say that states must marry opposite sex partners and make that relationship equal to natural ones because those peoples sexual desires define who they are as humans, like race or sex.(personally I think the House's defense of DAMA was very lame)
A constitutional based decision would be one based on the constitution, but it would also be a ruling that is easily understood on how it would-apply to different but related circumstances,