The Constitution gives the States, and only the States, the right to regulate marriage, with the proviso that the States will not regulate in a way that interferes with one's Constitutional rights.  See Loving v. Virginia.  

As for your friends and gay marriage.  For the past 50 years, secular and Catholic educators have failed to teach the Constitution, and Catholic educators have failed to teach Catholicism.   No wonder your friends think the Court got this one right.  Do you REALLY believe that the framers of the 14th Amendment thought that Amendment REQUIRED the States to re-define marriage so as to include man-man marriage?  Really?  When virtually EVERY State at that time (and until recently) had laws that criminalized that kind of "sex"?   You REALLY think the Framers of the 14th intended that?  A straight-faced answer would be appreciated.