I have too much of a strict personal view on monogamy that I would feel heavily irked every time they went out, because in the back of my head the voice would be saying, "Well, there she goes... Getting banged by a stranger again."
In my definition, I see no real point in being in a relationship with someone if there aren't things that are kept between specifically you two, which is, to me, always, sex. Polygamy, open relationships, prostitution/adult film acting.. Yeah, it would kill it for me.
There are times I wish I didn't have those feelings about it since I feel like a douche for having them, but regardless, that's who I am.