Guess it is normal in these times. Having more than one relationship is the norm derived from the fact that before marriage everyone goes through a number of relationships, mostly physical in nature while the two feel a fleeting love (?) for each other...which then becomes a trait kind of thing or say gets ingrained in the person. He/ she may stay quite in the beginning due to the charm of the new relationship but that soon wanes off and the developed trait to get cozy with others starts showing its head. However, not everything seems to be lost cos he sounds like not a very smart person to have let you know what he was upto and so when confronted may not do it again or may be would do to a lesser extent which then makes him a better option to keep sticking too. Another one may be smart, who would never let you know.