If it were me, I'd add a serious threat to their (ie, Tony and Anne) relationship via Mike and his wife.
If one is playing the swinging route, Tony and Anne can attend a party with Mike (a previous paramour of Anne) and his wife. Once upstairs, Tony is given the option of joining or watching. If he chooses to be the voyeur, there's a bit of an edge to how Mike's wife responds (can't remember her name atm). I'd play that out so that she takes it personal and at first wants to hold a grudge against Tony. That grudge would initially manifest as encouraging Mike to invite Anne over (she already knows how Mike feels about Anne and likely how Anne feels about her husband). She would encourage Anne to hook up with them on the sly, not telling Tony (as it would be more exciting this way). Once they hooked up a time or two Anne's charisma would also ensnare Mike's wife (I really need to remember her name) and the couple would then subtlety work to pull Anne away from Tony.
I like it....let's run with it.
