The opinion-starter already gave a good alternative reasons like physical abuse. I'll expand that a bit.
1. Abuse of any sort (not only physically but also psychological terror - did someone read Effie Briest?)
2. Mental illness that causes a partner to develop serious character changes making the partners incompatible or the relationship overall unbearable, especially for the person that has the mental illness.
3. Divorcing after the children hit adulthood to seek new goals or partnerships in life
4. Being financially irresponsible to the extend that it causes severe damage
The problem with divorces are not the reasons but that it often is extremely violent for the kids and between the partners. If we'd stop telling people that they HAVE to be together for ever and eternity, and tell kids that their families are not worth a thing, once their parents divorce, then divorces could be much more friendly and peaceful, ending in solutions like the dynamic divorces that allow kids to keep both parents while allowing the parents to get out of their commitment to the partner, but not out of their commitment to family. We make these things inseparable - they are not.
Many people are divorced over other things besides being faithful to each other! Wendy Davis divorced one of her husbands after paying off her student loans! Call that "unfaithful"? How about "let me mooch of of my some rich guy i married so i don't have to pay for my education", I mean really, is that really classified as unfaithful? The legitimate reason here would be deciding that you have no love for you spouse (this isn't against Wendy Davis, but just an example). Another example would be getting a divorce would be in the instance of them being absolutely disrespectful, and/or doing something awful to the marriage (examples may vary).
An example would be abuse, either of the spouse or of a child (children).
Another thing is that people may fall out of love peacefully and find other people they truly love. If that is the case, why not allow the couple to get a divorce?
While divorce is definitely not the best option, and people should take marriage seriously, unfaithfulness really isn't the only legitimate reason to get a divorce.
When you get married you take vows and breaching any of these vows is grounds for divorce (as you've been unfaithful to the vows, which covers far more than adultery) so many reasons that could be considered external to "cheating" would be things like abuse, reckless or out of control behavior, disdain, and even just a change in priorities and negligence to the marriage.
In the event of severe physical abuse, then I think that is a legit reason other than unfaithfullness, but technically, especially as a product of a broken home from divorce, I don't believe there is a legit reason to divorce. As bad as some things are that people get divorced for, I think divorce should be the last option