Communication is vital to any relationship, and I think it's even more important with being long distance. Signing offline without so much as a "I'm upset and need some time to think" is, well, disrespectful to him (even though I understand you were upset and didn't know what else to do).

I would recommend talking to him. While there may be some things in his past that you would never approve of personally, that doesn't mean the subs he was with didn't know about the others. I would not mark him a 'monster' without at least talking to him, telling him how you feel and what you'd want from him, and then seeing if he's able to provide that. Lots of people have open relationship and enjoy them - if everyone in his past situation was consenting and aware, then try not to hold it against him, even though it's something you personally never want. Make it clear to him you desire a purely exclusive relationship. You may be surprised to find that he wants the same. Or you'll find out he doesn't. But at least you will have tried.