I know one couple where the woman has been seeing people for a few years, and the man just started looking for dates recently. It seems to be somewhat stressful for him. What he HAS gotten out of this is an increase in her libido, from what I understand, which has been enough for him to not be out there looking until now.
If you are not happy with what is going on, renegotiate, if what you want is to be seeing others, SEEK them out. If you enjoy the company of the man she is seeing, that is great, I have never really heard badly about getting to know the people your partners are dating if you like them, and it can probably lead to much more comfort overall, and in the best case scenario, to great friendships.