This statement expresses a common sentiment among girls, indicating that they prefer to be with a guy who proudly shows that he is in a committed relationship and unavailable to other girls.
"No girl wants to be with a guy who can't let other girls know he's taken" suggests that women desire a partner who openly and proudly acknowledges their relationship status.
The phrase implies that it is important for a guy to be clear and upfront about his commitment to his partner, demonstrating loyalty and respect for the relationship.