What's the point of even taking vows if you were never going to hold your spouse to them?
Because marriages are a complicated thing and black and white doesn't always work.
Let's take my situation for example. My children's father was a very loving and loyal partner until our first child was born. All of his love and attention went to the children and I was replaced and he started detaching from me. If I see the marriage vows as either black and white, then he didn't forsake all others for me. He forsake all others until the kids came and I was replaced for his attention and love. I was given crumbs to keep me silent and the worse part is that I accepted being replaced because I felt guilty for wanting attention which he should of never taken away from me. He should of given it to all of us, not just the kids. Sadly, this is very common in my culture. The children come first and it should never be that way. That creates a crack in the marriage which will eventually get bigger and bigger that the spouses can't come back to one another anymore and a third party finishes what was started so long ago.
The ideal situation in my case was nipping his detachment in the bud, but I didn't see it that way at the time and he didn't either.
Honestly, cheating is a symptom that something is wrong. In my case the wrong started about 18 years earlier than when he cheated. We grew apart and there was no way he could come back to me once the kids grew up and didn't need him. He was vulnerable and easy prey for the third party to finish off what we both allowed to happen.
That is why cheating is not the deal breaker all the time. Now if the cheating is an exit affair (like in my X's case), then R is a deal breaker because the WS wants to unconsciously burn all the bridges because they are simply done. Humpty Dumpty can never be put back together again. Too much water under that bridge kind of thing.
And yes, there was illness in my first marriage, just like there is illness in my daughter's marriage as well. But there are other very severe issue too. Marriage is a gamble, and some win and others lose. There is no sure way to know if yours will last forever or not. Sadly mine was not going to be one that would last forever because there were too many issues that would eventually destroy it regardless of the love we once felt for each other. Love is not enough to sustain a marriage. Many things can alter its course and bring it to an end.