The theory here is that being the first to fall asleep could also mean that you're unconsciously trying to avoid a conversation relating to commitment. The researchers asserted that men may use sleep avoid these conversations because women tend to have better verbal abilities, giving them a sort of upper hand in discussions about the future of a relationship.
However, the study did point out that both women and men were equally as likely to pass out first, and not just the men. Most of the partners who didn't drop off so easily expressed a stronger penchant for cuddling and conversation after sex. The study's lead author explained this as:
"The more one's partner was likely to fall asleep after sex, the stronger the desire for bonding."
"If men actively avoid commitment promises in post-coital conversation, this could increase the likelihood of women ending the relationship ... Hastening sleep onset may evade this adverse effect," wrote the researchers.