Gaining

I think you're missing the part that rub people the wrong way. Near the end they say "There's no female members nearby to discreetly meet and start exploring."

As in behind there partners back.

While I understand being nervous about wanting to share, simply covering your ears going "lalalalala." And putting both himself and her through this is irresponsible.

Rather than taking the initiative and breaking things off when they are both dissatisfied op is choosing to let it boil and get worse and hope it just "happens." and that their partner will leave for them rather than doing the deed themselves.

There's nothing wrong with wanting to explore a fetish even on your own if it involves your body. It's your own body afteral, but when others get involved and it isn't communicated between both parties it starts getting shady.

And while op hasn't done such a thing, the fact that they desire thus and wont just let their partner go and be free or even fully express their desires is the part that others are condemning them for.

You should also consider some couples-counseling.

Most likely the counselor is going to tell them to split. There are probably a lot of things we would all like to do, but don't because our partners would object.

If he is older, she is going to end up having to take care of him if he becomes disabled or sick from his rapid weight gain. Gaining in your later years is a bad idea, medically.