In my turn I'd be interested in playing the man. The narrative tension feels interesting.
Who desires whom? - That's one of those things that could be fluid, or changing.
Have you, GS, got a notion of what sort of work the middle-aged couple do? I have a suggestion that the young woman works with one of the couple. She might be somewhat awestruck by them: perhaps they could be something in the arts, he the administrator of a theatre, say, the older woman an actor/director, and the young woman wants to make her way in the drama world. (But I'm up for other work-scenarios)
And perhaps this is something that's supposed to gee up their marriage after it's gone through a bad patch - but might in the medium term have the opposite effect?