“there is no provision to leave the Union”
There was (and is) no provision for federally mandated state sanction of sodomy. Is there any provision for the divorce of such sanction?
No, Lincoln and the civil war merely decided who had a stronger military. It did nothing to change that codified reality that Texas can leave.
"When, in the course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume, among the powers of the earth, the separate and equal station to which the laws of nature and of nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation."