Well - I know my Dad was involved with others - I think only women, but I don't really know. And after my folks separated (never divorced; never even legally spearted - just didn't live in the same house) my mother had boyfriends.
What I understood about both of them is that they really loved each other, really loved their kids, and couldn't begin to make a relationship work. And you can find all sorts of situations to blame it on, but nothing was simple or clear or easy.
Maybe it's a tragic flaw in my character, but I can't say that it changed my relationship with either of them. I think I just grew up really young understanding that promises are great, but that life is a lot more complicated than a vow you take at a single moment in time. There are so many things you learn about yourself as you go, and so many things you learn about another that it's a miracle relationships ever last.
And no matter what you promise, no matter what you swear and to whom, there is no promise strong enough to protect you from sorrow forever.