I want to express some skepticism about this term, when used to describe men who don't want to marry. (I realize this common usage was quite tangential to David's post; David's post simply reminded me of my objections.)
Commitment is scary, and should be scary. It indeed has all sorts of benefits, as well as risks — I should note that I'm happily married. But "phobia," which generally means irrational fear, is a generally unsound term to describe normal, reasonable fear of making what could be a very emotionally (and financially) costly mistake.
Nor am I just fighting the term's etymology here. As best I can tell, "commitment-phobic" is something of a pejorative in part because it still carries the meaning, or at least the connotation, of irrational fear rather than perfectly reasonable fear — or even perfectly reasonable quaking-in-your-boots terror.