The “secret” sloppy seconds experience is one I’ve always enjoyed.
I had an affair partner who often enjoyed girls-nights out, and from time to time would come back to our shared apartment freshly fucked, thinking I would be unaware - she would be horny as hell, have me fuck her and fill her used pussy with my cum, than almost always forse me into a face-sitting clean-up session.
My wife had several affairs, and would come home filled with cum, outrageously horny, and have me fuck her used hole, then lick my cock clean, sucking off all the mixture of cum and her own pussy juice. She thought I had no idea, but I am pretty certain I knew about every “guilt fuck” (a term she used later in our marriage, when she disclosed the affairs to me as part of counseling) she had.
I enjoyed the hot fuck sessions with both my mistress, and my wife, and the “99%” certainty level that they had had some strange cock, and that I knew, or suspected it, and that they thought I had no idea.