Watching your wife suck other people's dicks is a buzz kill. And she understands that by licking other people's testicles, her husband's boner will get sharper. So these swinger couples swap to sharpen their senses, bring back the novelty, and make their orgasms brighter. Only I would make the lighting not so bright, then there would be more understatement and less embarrassment.
Not only a mature man can get an erection from such a sight, but even an old man can get an erection from such a sight. He was very lucky that he caught the masturbating babysitter, because the girl is very appetizing and her pussy and ass just alluring with its narrow hole, into which I would also dive with pleasure.