I have this with my husband! We opened things up but sort of reacted to it differently.
I got involved with a co-worker and was unable to make it a one-time or casual thing, and so we've maintained things for about a year now. I sort of think of those two men as "my" people and have no real interest in having casual sex with others.
My husband, however, just has the occasional, rare, one-night stand.
Anyway, people do things their own way and how my husband does this doesn't bother me.