Sorry haven't been on here in a while and I'm just catching up to your somewhat busy thread!
I think you need to treat this current interaction with Mr. A just like you would in a new relationship. Wouldn't you want to know if you started seeing someone new and began being intimate with them that there weren't other people in the picture. I think if you are going to continue to be "intimate" with Mr. A then the exclusivity issue is a valid concern, both emotionally and physically healthwise. I think it's perfectly acceptable that the next time he comes over and the situation starts to turn towards intimacy that you are well within bounds of saying to him: "Mr. A. I really enjoy this newfound intimacy we are having, but for obvious reasons I need to know that the intimacy we are having is exclusive to each other, is that the case?" Clear and simple. If it's not then you need to question what his motives are.
