Is it okay or is it the biggest mistake?

So here's a question for all you out there why is it that when we have the seemingly perfect guy, relationship, and sex life we somehow get all paranoid if we are pleasing them. I mean if they are having as much fun as we are then when they say they need space for a little bit does that mean they want a break from the sex or a break from the whole thing? Then there's the fact that what if you are the one asking for the break and are in a relationship with no labels and aren't technically together and end up running into an old boyfriend or just friend you've always liked and sleep with them, should you tell your almost boyfriend or just count it as no problem because your not his girlfriend? I just wonder each side of the male and female's objective and opinion on the subject of "is sex okay with others when your not completely together with your man/woman?".