Sex shouldn't make or break real committed relationship as opposed to a booty call or flings. I feel that you should know and care for someone, before you fuck em. I'm a prude though!
Is there really a such thing as bad sex? It can always be improved... practice makes perfect
But if that's the most important thing to you in a relationship, then you shouldn't be in one. If you love someone, you should enjoy their company period and not need to go outside the relationship for sex.
Some people just aren't meant to be in monogamous relationships and should do the selfless thing and not hold anyone hostage in one.