I concur with the women Lefty has spoken with. Getting sex is not the problem. Getting "good" sex is. Add to that the fact that not every two people are sexually in tune with one another (e.g. just because you are a great lover to one woman may NOT translate into you being a great lover for every woman you bed...and the same is true of women.) Sexuality is much more than just the sex act and there are any number of factors that contribute to being good or not being good.
But having said that there are specific things you can do to get better at sex, starting with paying attention to the pleasure your partner is experiencing. Selfish lovers are without exception bad lovers. It doesn't matter how big your manhood is if you don't care about what sort of experience your partner is having.
And yes some women do factor sex very highly into whether or not they will marry someone. I know I did. In fact I stayed in the marriage much longer than I otherwise would have because the sex was always great. Why be married to someone who stinks in bed? Its a very important part of the marital relationship.
@marmel75 has shared some techniques for the sex act itself that I have to agree are good techniques. In reading some of his content I recognized a number of things my ex husband also knew to do (that we figured out along the way) which are anatomically reliable to get a woman satisfied.
There is so much male EGO tied up in how a man feels about his bedroom ability that I think the majority of men who actually stink in bed *think* they are good. And women are too polite to tell a guy he's not really all that great. Men have a really tough time hearing that. Men think size = skill. Nothing could be further from the truth. Size is nice if you know how to use it, but disappointing if you don't. And sex is not all about penetration. It's much more than that.