With the information you've provided, there is absolutely no indication that she is doing anything but enjoying you as much as you're enjoyhing her. So unless you can provide some evidence of red flag behaviour on her part, I can't understand why you're angsty? The only thing that would bug me is instead of texting the FWB back and telling him that she was in a committed relationship and he should leave her alone.. she just didn't answer him which leaves a door open at a later date. It was good she didn't reply but it would have been better if she shut him down. Ya know?
Anyway, You've had two other wives is there any reason why she should be having anxiety about you and them?
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion