For me personally, I don't have much trust for those who sleep with a friend/coworker and then get into a relationship. It shows that they can't be trusted to have a social circle since you never know who they're going to hook up with. It's not surprising that they repeat the same pattern and date other people in their social circle when the other situationship ends. Reminds me of "Friends" where they all just end up together.
If a person has a history of FWB, I'd question the dynamic of all their opposite-sex friendships knowing that they are always open to the idea. And you can bet that even if they aren't having sex, they are emotionally micro-cheating with at least one of their so-called friend. I'm uncomfortable with my partner sharing our relationship issues with his so-called close, female friend.