On the other hand, it sounds like your wife has a strong liking for being a "*****."
Regarding your own relationship with her, your best security may be that your wife's lover, in paying your wife to be exclusively his sexually, has the best of several worlds.
(1) He has his wife's permission to fuck your wife, so he doesn't have to pester his own wife for sex she evidently doesn't want.
(2) Therefore, his wife doesn't divorce him, nor does he have to divorce her in order to have a strongly-sexual younger woman to fuck him (your wife) — which would no doubt be a financial disaster; a divorce would probably cost him half his net worth.
(3) And, he's wealthy enough to pay your wife for sexual exclusivity — an arrangement your wife accepts — so he doesn't have to worry about acquiring an STD, which could be something incurable and extremely serious (like HIV) and/or potentially fatal (like hepatitis B).
Apparently the only disadvantage in this arrangement is to you, in the sense that you have to accept being sexually denied by your own wife (or so I gather).