Women want to feel special, like *she* is the special one who tames the wanderlust of the philandering male. So, winning his loyalty to her after having been with a number of partners is flattering.
Men (at least those who want to pair bond, which is most, apparently) want to know that their woman is loyal and selective. So, any children she has is more likely to be his and not some other shmoes.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh