People explore polyamorous relationships for a variety of reasons so it's difficult to say what's an excuse and what isn't. It's not uncommon for people to arrive at open relationships or polyamory through some act of cheating. Sometimes it works out with a lot of work, other times it doesn't.
For what it's worth, I don't choose additional relationships because of things that might be "missing" or "lacking" in one relationship. Every relationship is whole for me and I wouldn't be getting into any relationship that was great in some ways but lacking in others. I find most poly relationships thrive if each relationship is whole and healthy. It would probably be a good idea to address that first.