What we are talking about here is what the couple should do if one of the partners loses all sexual dsire and ceases to enjoy having sex. That can happen for any of number of reasons. If it does happen, I don't think the desire-less partner has a moral obligation to have sex with his/her spouse.
In that case, the desire-less partner has a moral obligation to accept an open marriage or suggest divorce, because they don't have the right to expect "celibate faithfulness". That's just reality -- you have a moral obligation to embrace a choice. Otherwise what you're saying is the desire-less partner has a "right" to just duck their head in the sand and pretend they're in make-believe land.
