How important is good sex to a relationship?

i agree with DC(physiological) i think he just doesn't want to fuck you (or cause you hurt) and his mind strays to someone/something else or fear/guilt sets in and BOOM deflated intentions...try tying him up to releave him of the control and ease his mind that he is doing it to you, instead you are taking what you want....then he can relax that "he is hurting you" mind over matter

Sounds like a psychological issue. Could be psyching himself out.
Needs to relax and not treat sex like a job.

Good sex is important. Like you said, if you don't get what you need in those areas, you go looking somewhere else. If he is hard until the point of penetration, then I would say he has a psychological problem that needs to be addressed. It is frustrating to be in a situation where you really do have feelings for a person, but aren't getting what you need, and really don't want to hurt them by straying. It's a choice between hurting them, hurting them, or getting over the problem. And if you can't get over it, then hurting them seems the only choice. Kind of between a rock and a hard place. Sucks. sorry

yeah, is he a really big guy? perhaps he's dealing with issues of hurting lovers in the past.
you can try a few approaches. such as getting him on his back and making out with him. rubbing your vangina on his penis and slowly pentetrating yourself with it while he is preoccupied with making out with you.
you can try sex toys, cock rings and such. make it out to be fun and entertaining.
or you can sit him down and have a heart to heart with him about how your feeling, how much you love him and what your wants and needs are.

somewhat

somewhat