Here's the thing that people have touched on, but no one has actually said.
A woman's base instinct is to find a strong male to protect her children. A man's base instinct is to plant his seed.
Women feel the biological clock ticking, and men feel pressure from the knowledge of it. When we start to think about committing to a woman, it all looks grand on the surface, June Cleaver baking and vacuming in a dress , and all of that. Then we see ourselves going to work, and coming home to a dull life. We've all been through this before and know that those little quirks that were once so cute can grow into HUGE annoyances. We get a bit scred of that emotionally, because we don't want things to change for the worst. If they are becoming annoying, so are we, and there is always at least a small insecurity, saying anything else would be a lie.
So instead of dealing with the no longer cute now annoying bound to fight and ruin it all, we have a fall back. PLant seeds. Hell I'm just following my base instinct. I'm a guy, it's what guys do.
I guess the thing is, if we stop growing together, we grow apart. That's where the fights start, that's what we are really afraid of, waking up one morning and saying, who the hell is this and where is the girl I married? The worst part is knowing the answer, she grew in a different direction.
For me the answer was simple and yet hard to swallow, grow together. Always grow together. It's one of the reasons that I left the FS communities nearly 4 years ago. So that I could grow with her. Now i'm back, and things are better for me both here, and at home.