It is not about who needs who, it is not like we are running out of tomatoes here and we want to make a salad. Ok go ahead and buy some from the grocery. The relation between men and women is simple and complicated in the same time. It depends on the desire sometimes as Cornellian said earlier. What do you want out of this relation can determine if it would a successful one or a failure. If it is a real relation or a fake one.

Answering this question from another point of view would be that no one can live alone and we all need each other. Men and women naturally need each other and I'm not talking about the biological needs, but the emotional needs which is more important than the biological needs. The relations built on biological needs never last and are shallow.

No one can deny that we men came out of women and would always love to return back to them in the end. Honestly, no matter how much older the man grows, he would always be his wife big child.
A woman also would feel the need for a man, to be always there for her, protect and support her. She got emotions and feels that she wants someone to share that with someone, but she needs to feel secure to share those emotions with a real man.

I can't talk about the need for having children that everyone got their deep inside, no one can see those little angels and say: I want to get married, but not to have those kids who would prevent me and be a stone in my way to have fun in life. All of those people forget about all of the comments that does not make sense before when they got their first children. Try reminding them with the past and you will get answers like: I was a fool, I did not mean it. To make it simple the need for having a family is a very good reason for the women and men to need [love and cherish] each other.

Some are afraid of being lonely when they get old, so we got this kind of relation after the kids [used to be] get married and the visits are less frequent. Should I spend the whole day alone? Even some of them don't declare that or say it, you can feel that.

The man by himself is not complete, he is only representing a half.
The woman by herself is not complete, she is only a half too. Men and Women are living their lives looking for the other half that makes them complete.

Best Regard