My boyfriend was a virgin before he met me?

I definitely think you should be worried. He is obviously very caught up with the fact that he has not had a chance to have sex with other women and may, if he marries you, never have that chance. Those kinds of unresolved feelings are what break up marriages a few years down the road. Not talking about it won't make it go away and putting a wedding band on his finger certainly won't either. Kissing someone isn't the end of the world on its own but it does indicate that he has the potential to cheat and it's on his mind. Unless you want to be 25 and divorced a few years down the road, make sure this is resolved before you make a long-term commitment to him. Either let him go do what he needs to do (have sex with a couple girls) and hope he comes back to you or make damn sure he has gotten past this before you even consider marrying him.