For me, sex is the cement that holds a relationship together. There is nothing much worse in a relationship than being sexually frustrated and your partner just not giving a shit how you feel.
I'd give a partner some time to improve, but I'd want to see proof that he was not only working on it, but improvements quickly.
It's one of the good things about not waiting too long to have sex in a relationship. If the person is really selfish or awful in bed, you haven't invested too much in it, and can move to a more satisfying relationship without a lot of drama. Put too much effort in the relationship before
sex, if the sex turns out to be terrible, and you've wasted months or even years of your life, not to mention all the emotional effort you put into it.
Fuck early, fuck often and fuck well. It's the key, as far as I'm concerned.
I'm glad to NOT be in a relationship where the sex is frustrating or nonexistent, I wouldn't tolerate that for long. The sex is amazing and always has been, from the beginning. We both agree that neither of us could stay in a sexually cold, sterile or selfish marriage or relationship.