Sounds like he is not too concerned about what your reaction is, which means he may be already distancing himself from you...not unusual, if a guy is "in lust" with another woman. Try not to overreact and don't compromise your own morality, just ride it out with as much courage and good grace as you can muster and hope that your good judgement will show him the real way to a happy future...That's how my wife handled it and now I love her more than ever...(this was years ago)..Good luck.
If he is cheating on you know what says he wont in the future? You need to confront him about it. If your sure he is cheating then you can accuse him of it but if you don't know you just think you have to be very careful about how you do it. I would advise only giving him one chance. If he cheats on you again, leave him and don't look back. If you children come to find out what he is doing they could become the same way in their relation ships.
He seem to be cheating but ask him about the girl or call the girl let her know why is she called you boyfriend talk to him if he does what to worry about maybe you need to show him the door. What is a relationship with out trust and honstie. Once you lose that you really can't go back to the way it was. Talk to him see how he reacts that should tell you a little of what happening.