Women really do like humor, but something else might be at play here.
A scientific study came out recently that adds support to the hypothesis of "the wedding ring effect" which surmises that women are far more attracted to men who have already been taken.
Women have a hard time, deciding on their own, whether a guy is attractive or not so they look to others to see if they share their opinion. If a guy is already taken, women think to themselves, "This guy must be desirable because someone is already with him." Otherwise, women are not quite sure. They must be cautious because there are so many weirdos, rapists, and stalkers out there. They lack confidence in their own judgment.
Many men notice that as soon as they get a steady girlfriend, other admirers seem to come out of the woodwork.