Yeah, I don't really get it either. I'm not sure how you are doing things but I know with my fwbs if one of us starts going out with someone else we stop having sex with each other until we are single again. If he is constantly talking about having whatever with other girls and you're not caring about it then why let it mess up your 'good time'. Then again that could just be an opening in him trying to get you invovled in a three-way.
"I saw myself as he saw me and the amazing things was that in his eyes i'm beautiful." Liz Parker-Roswell