I think a lot of us got distracted by the horny wife. Going by the pages backwards, I noticed that he said: "If, by the time you see her again... she were alone and helpless".
If he knows his wife is a nymphomaniac, it actually makes sense. He loves his wife, and the choice is either have her suffer or get it on with another person. When it comes to the latter, of course he'd prefer it with someone he trusts instead of a random person. He would be a terrible husband if he actually prefers the former. That would be more akin to possessiveness instead of love.
Not something people think about unless their days are numbered though.