It depends on the feelings of the man.
If a man is "in love" with his girlfriend then he is not going to be (more) in love because her title has been changed girlfriend.
From society's point of view it's a "promotion" in relationship status when a woman goes from being a girlfriend to being a wife. Many people also view marriage as being a "commitment".
Commitment actually comes (before) there is a marriage!
Not many people would choose to marry someone who was cheating on them, lying, or untrustworthy. The exchange of vows just makes the promises/commitments they've already made in private a public statement. A marriage decree entitles the couple to certain (legal and financial benefits) especially if the marriage fails.
Kurt Russell and Goldie Hawn have lived together for over 30 years. By most accounts they have a loving and monogamous relationship.
Bill and Hillary Clinton have been married for over 38 years. Very few women would want a boyfriend let alone a husband that slept around like Bill. The vows mean nothing if there is not already "a commitment in the heart". How your mate (treats you) is more important than what your title is. One man's opinion! :