A WG I had seen a few times, and whom I continue to see, invited me to participate in a threesome with her hubby. Really wasn't sure at first, but curiosity prevailed and I went for it. Both me and the hubby are straight. She's not.
We took her to local dungeon as a surprise birthday treat; all three of us are into BDSM. Had a brilliant session and, to cap it all, no money changed hands as she didn't consider it as 'work'.
Since had a foursome, mfmf, again great and no charge.
The hubby just seems to really get off on seeing his wife getting banged. Each to their own.