That's a fair point. I think I originally told her "I will not pay for you to have a cell phone with which to carry on an affair," and also "You are not to talk or text him in front of the children." I may have said "in front of me" as well, but she doesn't really do it right in front of me, she will go in the bathroom, or wait until someone leaves the room, or whatever. She also gets a lot of text messages from D20 and D18 since they moved out -- esp. D18 -- so if I call "b.s." on this, I'd better have damned good proof, since it will require checking her text messages.
But it's coming. In fact, asking her to just plain MOVE OUT may very well be coming, if her behavior doesn't change. But I will have danmed good evidence when I do that.