Does being in a relationship affect whether you hang out with other couples or single people? Does it bother you if your significant other hangs out with their own friends without you?
Quite the opposite; I'd be bothered if they didn't. We were both self-sufficient people with our own interesting lives before we met; that shouldn't end when a relationship starts.
Absolutely true, no one single person can be everything to anyone else. It's a common mistake in relationships and marriages. It can be quite suffocating.
^ this stuff. I have no problems with my girlfriend doing stuff without me, and I sometimes like to do stuff without her. But also I had friends before we got together, and I still have friends. Whether they are dating someone or not, they're still my friends. So, it depends, but not on my preference of whether to hang out with couples or single people, but on whether my friends happen to be dating or single at the time...