OP,
Did becoming a sedevacantist destroy YOUR marriage?
How? Are you the sedevacantist? Or is your spouse? What is the "status" of the non-sedevacantist spouse?
How exactly is the marriage being DESTROYED? That's a pretty strong word, you see. There's a difference between a momentary or periodic disagreement (however severe) IN a marriage and the DESTRUCTION of a marriage.
Becoming a sedevacantist did not destroy my marriage. Eventually the other spouse became one, too. Pretty quickly, at that.
Becoming a sedevacantist will NOT destroy a marriage, because it has no spiritual impact. Becoming a sedevacantist doesn't mean you stop making a morning offering, stop saying family prayer, stop going to mass, only go to masses said by sedevacantist priests, etc. NONE of these things follow from believing the chair is empty, so if your marriage has suffered SINCE coming to hold this position, it is not BECAUSE of this position, it's because of something else.
The only exception I can see is if the non-sedevacantist spouse isn't a Catholic to begin with, or a thoroughly Novus Ordo Catholic. In that case I can see it having a detrimental effect, though the detrimental effect should have already been there if you were a traditional Catholic and your spouse wasn't, and the situation is merely exacerbated by you taking this position.
Summarily, if a man and wife are traditional Catholics and one becomes a sedevacantist, this will not "destroy" the marriage; any disagreement (however severe) resulting afterwards is the result of one or both spouses misconceptions about the position, and thinking it requires something that it actually doesn't.