If relationships are as great as you yourself advocate, why limit yourself to just one? Hmm?
To me, fukking somebody doesn't mean you're in a relationship with them.
Besides, like I said, I'm selfish. All I need/want is one person to be as selfish about me as I am about her. To me, you might as well stay single if you feel the need to have multiple sex partners.
Unless, such as your case, it just so happens to work for your dynamic. I know myself enough to know that, at the risk of sounding possessive as fukk/as if a woman is property (which isn't the case), I don't like sharing like that. I barely tolerate sharing food off my plate, so I know it wouldn't work with my girlfriend.