Goodness--of course marriage and a PhD can go together, without harming either one. OR they can be a terrible combination. Much depends on the approach of the 2 people involved. At some point in life--whether it is the PhD, a career, a child, an ill parent, or whatever--something may come along and make demands on the time people have to spend together vs apart. Inevitably there will be something. A strong relationship finds a way to endure these--perhaps is even made stronger because of them. A troubled relationship may get unhinged by them--but it is unlikely that the event all on its own is going to be the unhinger. Stresses just point out the "weak" or "strong" points in the relationship.
I pursued my PhD because I got divorced! I felt I needed to do something productive and positive with my life, to move ahead to the next thing, and a PhD was as good as anything else to do this... even at the times I was not convinced I wanted to be doing the PhD, I used to tell myself, well at least its doing SOMETHING!
Would my own marriage have thrived or survived on the PhD? Hard to say. My ex and I actually came together very well in times of crisis or strain, so perhaps the PhD would have been very possible if I had tried to do it whilst married. Who knows, perhaps it could have brought us close enough together to be able to resolve those things that did ultimately end the marriage....