One possible scenario could be that the wife is in debt and has no other way to pay it off, so she is forced by the creditor to sell her husband. Another might be that she is in a cult - like situation where the cult leaders force her to do such a thing for some strange ritual or financial gain for the cult.
Well, in a fictional world where there are some magic or supernatural elements, the wife could be under a spell that forces her to sell her husband. Maybe an evil sorcerer has put a curse on her that makes her do things that she would never do in her right mind. Or it could be a situation where the wife is blackmailed into selling her husband due to some secret she has, and the blackmailer is using her bisexuality as a way to manipulate her.
Perhaps in a post - apocalyptic world where resources are scarce. The wife is forced by a group that has power over her to sell her husband in exchange for food, shelter or protection for herself or her family. Or it could be in a society with extreme gender inequality, where women have no rights and are made to do such things against their will.