Ive been happily married 20 years (3rd wife).
But lately much younger women are coming on to me, half my age in mid-twenties. And tbh I'm very tempted to switch to a younger lady...
Then you are not "happily" married. You are just tired of her pussy, as you have gotten used to it over the past 20 years and want something new.
But that aside, why not just ask your 3rd wife if she'd be into an open relationship? You know, where you both can fuck who you want but still remain happily married?