I noticed this at The Wendy Cooper Weblog:

...once they have life all figured out, things will fall into place in the relationship. They have forgotten that relationships also consist of two people and totally ignoring the other person in the process. I don't know why that is because they insist they they value the other person but has a strong line of separation between them and anyone else.

I think the problem (especially for us men) is that you have to start with one person first before you risk sharing for two.