I have had sex with couples. I have had solo sex with the husband from one of those couples after his wife started getting weird. She was always the one to invite me over. The threesomes were her idea. I just wanted to have sex with her, but she wanted her husband involved. She insisted. When it first changed, she was still the one to invite me over. We'd all go out, see a movie or a show, maybe get dinner. We'd get to their house, and she would evaporate. She'd be nowhere to be found. or, she'd get online and ignore us. We' d start making out, and try to lure her into the bedroom with us. She'd tell us to start without her, and she'd be in soon. But hours later, we'd be done, and she'd still be online. It got awkward for me. So the husband started inviting me out with just him. He said she knew. He said it was her idea, and that she was busy with the kids. She wasn't speaking to me much, so there was no opportunity for her to contradict him. Years later, after they divorced, she asked me if I had ever slept with him without her. I knew then that he'd lied, and she didn't know. So I lied. I don't feel good about that, but I do feel that it was less painful for her that way. She thanked me for being a good friend, which of course, made me feel even worse. We never had to be behind her back. I could have fucked him with her permission, any time I wanted. I wish none of that happened.
The other married couples were drama free. I met with them a small number of times, and then stopped hearing from them. That is probably the best way to go about that sort of thing. No one gets too attached, no one gets jealous.