What do when your boyfriend sleeps with another girl?

If he's falling for another girl you should ask yourself whether YOU still want to be in the relationship. I'm a guy and I will tell you that if he hasn't taken the time to talk to you about what is going on and given you the option of breaking up with him he's not a good guy. People's feelings change but you should always respect the person you are in a relationship with. If he's getting involved with someone else while the two of you are still dating he doesn't respect you and you don't owe him anything. I know it's hard but the best thing you could do would be to dump him and find someone else. Never stay with a guy who won't be loyal to you. There are plenty of guys out there who will be.