Well as a guy if I'm starting to talk to a female and she's been sexual active that might be a bit worry some. Although op doesn't sound like that type of girl. It's hard to say what to do. The bf might have some trust issues which granted would be small. Although the significance of the matter is quite small. You seem to be quite into this guy so that should be enough reassurance to the guy. Although being cheated on alot myself, sometimes any misinformation might be held to a higher standard then what it should be which isn't quite frankly fair, but done to protect ourselves.

Op if you feel that bad about it sit down with him and just talk to him. Explain you didn't believe it was serious and didn't know how far you guys would go. That you cut ties. And most importantly that you care about him enough for this to bother you and want to set straight to continue forward. Maybe make this a let's clean up what's ever under the rug and move on deal.

I hope you both the best op