Is Bad Sex Cause For A Break-Up?

Sex is plays a big part in pretty much everybody's relationship, whether they've decided to abstain until they're married or they decide they can have sex whenever they feel, sex is usually a topic of discussion in relationships. Here's my question though, is sex such a huge part of a relationship that you'd end one if your significant other just couldn't cut it in bed? Is it so important that if you had sex with someone before making it official and you didn't enjoy it, they'd never become your guy/girl? If you really care about someone shouldn't you be willing to work hard to make it work even if that means showing them how to please you in bed? I personally think that if the person has everything you want and need outside of the bedroom, you should be willing to at least try to make things better sexually. Who knows? It might bring you guys closer together. And as far as how to bring it up without hurting the person's feelings, just be honest, make sure you approach the issue lightly and choose your words carefully. But then again, it's kind of hard to say if you've never been in that particular situation... Have you ever been in that type of situation? Do you think bad sex is cause for a break-up? Should you try to make things better sexually if they're great in every other category?