A wife has no obligation to provide sex on demand; neither does a husband. Same for girlfriends, boyfriends, mistresses, gigolos, brothers, sisters, and random strangers. And anyway, legal entanglements and promises to love, honor, and sexually satisfy aren't the issue. The issue is coercion.
Nobody here is saying that anybody has any obligation to have sex. We are just saying that lots of people in many situations also have no obligation to provide money or support. If they offer support or money (that isn't owed) in exchange for a service, that's simply called doing business. Why should it be deemed coercion if that service is sexual?