I would first ask who was telling the story. If the husband is telling it perhaps he just got some bad news from the doctor, and he wants to find somebody to take care of the wife with her unique "needs" going forward. A slightly depressing subject for an erotic story, but it could work. Maybe not death, but perhaps masculine incapacitation?
Is the husband a good guy, does he want good things for his wife that he can't provide; or is he somebody who gets off on demeaning her, "forcing" her into a situation that she doesn't want to be in? Does the wife want a baby, but is the husband incapable of making one? That makes this man she's dating little more than stud service, and choosing that man for his physical and mental attributes then becomes critical. Was there a study of "available" men, and was this the winner?
If the wife is telling the story it's a bit more of a challenge, (she can't be ignorant of her motivations) her being married and out on a "date," (while dressed for attention, with a remote device inserted) screams "I want sex." Being a teenager and accidentally teasing someone, or even doing so intentionally to twist somebody up, to experiment with your sexuality just because you can, is different than a mature married woman doing so after one failed marriage (assuming she's not a widow). What happened to the first husband/marriage becomes a big question here too.
Does the wife actually want to be on this date? It could be my interpretation of your scenario, but if feels like she's doing this for her husband and not herself. Did she approach her date, or did he approach her? Is her date married himself, and is this actually wife swapping with her the only one of the four that doesn't know this?
In the real world my husband may offer a hall pass, but he's not telling me who and how I'm to date anybody, to me that kind of drifts into pay for play... There's a thought, is there a debt between both men, (as in Indecent Proposal) and is the wife the coin that settles the debt? If that's the case though, it might be nice to let the wife know this, give this submissive character some rules to guide her. Opportunity without "permission" is temptation to me, but a great story element too, lots of tension there, so maybe this would work for your characters.
Is the boyfriend a character of depth, or just a set piece stiff member looking to score with a married woman from the office; or perhaps factory even? Has he secretly lusted for the wife for years, or is she a "she'll do" for a romp because nobody better looking is here tonight that will talk to me? One is hugely empowering, and the other crushing to your soul should you find out.
Did the husband recently "cheat" and is he setting the wife up to "cheat" too, so that they're even now?
I'm afraid that I've given more questions than answers, but I hope this helps, best wishes, Jackie.