It depends on what you want out of a relationship and the kind of person you are.
I'm the kind of guy who's fairly capable but unmotivated (I've gotten over the whole "I'm not as good as" High School syndrome.) When I'm single, I just laze about all day and don't accomplish much; the bare minimum for whatever it I'm doing. However, having a girlfriend gives my life structure. I wake up and know that I have a date in a few hours so I ned to get my self together. I know if I don't do my work my girlfriend will be dissapointed in me. This is just the kind of person I am. I want someone in my life to share my experiences with and to keep me motivated. Some people don't need this, I feel I do. If I could give you any advice it would be to find someone who's just like you in most ways. Ask them out on a date and see how it goes.
On the subject of sex, sex is a human need and it's basicly addictive. My girlfriend and I have sex fairly often but I still look up porn frequently when we can't be together. Everyone puts a different level of importance on sex, in their lives and in their relationships. To me, sex is very important in a relationship but not in my life. That is, when I'm with someone, we should be having good sex, otherwise something is wrong. On the other hand, if I was single, I am not going to risk my life just to have sex. However you feel, if you're in a relationship or looking for one, just remember: never underestimate sex.