Because fathers are less of a presence than mothers in children’s lives, fewer models are available to boys; thus boys come to define masculinity as “not being feminine” or not being like their mothers.
This reminds me of something that was brought up earlier in the textbook: men are less likely to or more stigmatized for stepping outside of male gender roles than women are for stepping outside of theirs. I remember that the earlier explanation was that since men have more power in society, it is less acceptable to reject the higher-status role. However, here it seems like another reason may be that men define their entire role in opposition to the female gender role. Instead of building their role from the ground up based on a role model, they first learn it as the opposite of femininity. Therefore, it would make sense why men stepping outside of their role into femininity is considered less acceptable.