I'm by no means a doctor, but from what I understand, you would only need to worry about an infection if dh had some kind of infection that he could have passed onto you. If that were the unlikely case, the infection would be worse just bc your cervix may still be open from the m/c. I think you're probably ok though.
As far as sex goes, my doctor told me to wait until my hcg levels were at 0 before dh and I "resume relations" (lol), which she said will likely be next week. I have no idea why we have to wait though!