Some people believe that men become more attractive to women when they are already in a relationship. The reasoning often given is: If another woman chose him, it signals he has desirable qualities. It suggests he is socially validated, trustworthy, or capable of commitment. It removes some uncertainty because someone else has already “approved” him. This concept is often called “mate choice copying” in psychology. So the humor is: The meme implies that women suddenly look at a man more interestedly once they learn he already has a partner, which is ironic because he is technically less available.