Its not just men. Women can have the same issues and still back out of the relationships. Some women won't stay in a relationship if they get pregnant before marriage because they worry the guy isn't really interested in her, but just doing what he's 'supposed' to. Some women choose to get abortions, even against their boyfriends wishes, which can destroy a relationship.
But pregnancy can even be hard on marriages. Anytime you bring a child into the picture it runs the risk of complicating things, jacks up the expense, puts pressure on the people involved. It alters peoples priorities and commitments, and some people don't like the changes that brings about. Married or not.
Pretty much anything that is a major change within a relationship, having a child, major changes in careers, financial problems, etc.. can put stress on a relationship.