The first 2 answers are awesome.
As someone that had a loving (poly) relationship with a married woman, I think a new couple should decide the limits of emotional bonding first. Then, plan backwards from there. It won't be a strict narrow answer, but more a generalization.
Every wife and Bull with have different abilities regarding compartmentalizing and keeping it to the FWB level, if the FWB level is the goal AND boundary.
Some people can meetup weekly with the same FWB, and not form romantic feelings, others can't and should meet less often with the same FWB. Just having a "thou shalt not" rule is not enough. I think people need to just meet less frequently with the same person, unless that Bull is good at keeping it casual. Usually Bulls and wives with multiple lovers are better at this. Some can even sleep over and keep it to the FWB level over years, but many can't. You all need to play it by ear and consider what the original boundaries were.
Boundaries can be renegotiated in the future too, but not during play time. It's best to do that with a cool not-so-horny head.
I think too many newbies think just a rule is strong enough. Nope! Consider frequency and whether over nights are practical or not.
I think having a honeymoon period for the first couple of weeks is hot, and fucking several days a week. After that though, they'd be baking a relationship cake if frequency was not reduced, probably.
