Having been the one that was depressed, suffering from PND, I would say yes it is wrong.
You cannot believe the hurt and distress this can cause your long term partner/ husband / wife that can be ongoing once they have climbed out of that big black hole they find themselves in. There will be no way to hide it, the secret will always come out not necessarily straight away, but along the line.
You would honestly be better leaving your long term relationship and moving on. It is easier to deal with a complete end to a relationship for whatever reason than to leave your current partner feeling dirtied by your actions
My fat balls bring all the birds to the yard
I'm not a pheasant plucker....
