They would be together with their spouse and be able to talk all they wanted and have sex when they wanted and live in an unencumbered world together.
In one situation, I did mention to the man in question that I had a boyfriend I referenced him early on in our conversation , I didn't realize he hadn't heard and only realized it when he emailed me a few days later asking me out.
Why would you hide something like that from your FWB considering that your relationship is just FWB? She is not a priority to him.
The fact here is that if you love someone you should trust them with the truth whatever that may be.
Either from within the relationship, which is more common, and someone comes along that fills that need.
Yell, Hit, Put you down, Bully, you name it, withhold love and affection when he is pouting, and the list grows.