Realistically, i'd say Kakashi simply isn't interested in a monogamous relationship, and/or hasn't found the woman that fits his requirements. I may sound a bit misogynistic, but after the conclusion of the Fourth Shinobi War, which was when all the hooking up started, Kakashi was pretty much the best man that a woman could get, a regular James Bond that pretty much has no flaws, so it's kinda safe to assume he could have had any kunoichi, if he wanted to.
I think Kakashi just concluded that he'd prefer to experience "Icha Icha Tactics" first hand, but not have a longterm relationship.