I don't think this is that uncommon. People change, and couples that were once in a passionate relationship drift apart. In this case you have three choices.
1) Stay committed in a sexually dead marriage. This means denying your own needs for sexual intimacy.
2) Satisfy your sexual needs with other people, but stay in the marriage. A lot of people do this, and if it is done in the open I don't see any problem with it.
3) End the marriage and move on.
I personally would find option #1 awful. I don't see why anyone would stay in a marriage with someone who no longer cares about meeting their needs.
My divorce was one of the best decisions I have ever made in my life. I was stuck in a marriage that was dead sexually. That was a part of it. Divorce freed me to find someone who wanted to be in a giving sexual relationship.
I think that seeing other people in an honest upfront way is perfectly reasonable if there is a reason to stay in a sexually dead marriage. I don't see the problem people have with this option. It is an honest and caring. It keeps whatever part of the relationship that is still working intact. And, it allows everyone to get their needs met.