That, of course, is a matter of "internal" Church decisions.  However, it's extremely presumptuous for you (and, if they contend as do you) to assert that "Christian Churches and Christians CANNOT AND DO NOT sanction same-sex marriage."  Obviously, thousands can and do.  You only counter-argument is to contend that by doing so they are, ipso facto, not TRUE  Christian Churches....an argument I expect you to make.  I feel your position is the height of presumptuousness.
Go to the link I provide and see the very long list of Christian Churches which sanction (bless) same sex marriage.  regards, Gary Crum