From, The Science of Kissing by Sheril Kirshenbaum.Since a man's saliva contains his testosterone, swirling his tongue into a woman's mouth is a way to legally slip her a natural sex stimulant. Over weeks and months of kissing, scientists theorize that she may become more interested in getting physical. It's not something that would happen in one night, but as her suitor persists over time, the influence of added testosterone may have a cumulative affect. This provides ample reason for a man to continue pursuing the woman he's kissing and also explains why men are more likely to view kissing as a prelude to sex, and to report the preference for more tongue. While women may not enjoy a sloppy kissing experience as much, it can give men an added sexual advantage, and has probably been a successful strategy throughout history.
Pucker up, boys!