Compare the following cases:
Cavaradossi has (justly, we may suppose) been sentenced to execution. His lover, Tosca, begs the magnate Scarpia to release Cavaradossi; Scarpia agrees to do so, contrary to his oath of office, on condition that Tosca sleeps with him; she agrees to do so.
Chuck has a strange property that any woman who sleeps with him will shortly after meet her true love. Reba agrees to sleep with him, in order that she might obtain a husband.
It seems to me that Tosca is exploited whereas Reba is not. Yet the trades that they are offered are identical: sleep with a man who they would not otherwise sleep with, and gain their desired romantic partner.
What might explain the difference? Perhaps we may think it is that Tosca risks losing her lover, which is considered worse than merely failing to gain one. But Tosca has no just entitlement to Cavaradossi, since we have assumed he deserved his sentence. This trade is therefore more properly viewed as a gain to her, rather than merely the avoidance of loss.
There are probably more explanations. The problem is that there's one - see below - which upon further thought seems like it might explain the difference, and I can't think of any others because I wrote everything up to this paragraph earlier, but now I'm TOTALLY PISSED AND EVERYTHING IS EVEN MORE AWESOME THAN USUAL BUT I CAN'T DO ANALYTIC PHILOSOPHY AND CONTINENTAL PHILOSOPHY IS BELOW ME EVEN WHEN I'M DRUNK. AND YES, I JUST COMMIT MYSELF TO THE VVIEW THAT THERE IS A "ME" WHICH PERSISTS OVER TIME.
Perhaps it is that Scarpia could release Cavaradossi without sleeping with Tosca, whereas Reba's sleeping with Chuck is necessary for obtaining her a husband. But this is simply a matter of how we have constructed the thought experiment