The best outcome.

“If you ruined someone’s silver tableware, you must repay with gold tableware. The president of a polluting manufacturing plant must live beside the plant’s chimney. That’s what taking responsibility means. However the people in so-called high positions never see the consequences. When mistakes are made, they will casually bow their heads and apologize. They think that’s enough. Are you one of them too?” – Toua Tokuchi(from One Outs)

This situation was a bit different but in one of my favorite manga/ anime one of the main characters let a disgusting old man fuck him for money. He did this because it was the quickest way to fund his band of mercenaries.

I don't think that I could go that far to pay of my debts. After letting some man use me I would feel really bad. However I would fuck a decent looking girl if she would have lent me money/if she would offer to give me money to pay back my debt to someone else. And I would likely accept it if a girl who would borrow money from me would offer to fuck her instead. You can't get all the money so I may as well fuck the girl if she wants to settle her debts that way. If she would not have the money I would not get it back anyway so if we both would be willing to have sex that would seem like a best outcome. But the girl would have to be the one to suggest this. I don't think that I would ever ask for sexual favors as payment for debts. That would feel wrong. But if a man would borrow money from me I would only accept money and not sex.

So in conclusion I don't see anything wrong with paying debts using sex as long as both persons are willing. At the same time I do think that giving money and hoping to get sex in return would be kinda messed up.