Please don't hide from A that you are seeing R as a friend. You think you are protecting her, but you are thinking for her. You've done nothing wrong. You have nothing to hide or be ashamed of. Tell the woman you love you had a friend over for pizza. Nothing more nothng less. If you want a serious relationship with A, I strongly encourage you to be honest with her.
A.'s jealousy is for her to deal with. The best thing you can do to help her deal with it, is not hide stuff from her. She will find out at some point, and then she will no longer trust you - and rightly so.
I've been reading your blog and rooting for you.
early forties, straight.