I'm the mono husband of a Poly wife and it was also unexpected. Even when things are going well, it can be enormously difficult to cope with some days. My sympathies.
Addressing your question, the idea seems to be "I feel our relationship is slipping because of her other relationships, so I'm wondering if I should have relationships myself.". That sounds dangerously close to "Relationship broken, add more people." I've thought that dating someone just to "even" things up would be disrespectful to the new person as well.
I think it might be better to work on improving things between you and Wife. What things do you feel are slipping? What do you feel insecure about? What actions or words might help alleviate these feelings?
Just my thoughts