When we stepped into swinging initially, it was strictly for sexual exploration. We didn't have any outside contact with the couples we met up with, other than to have a drink or dinner as a precursor to ending up in bed with them. It was straight up, casual, no strings attached sex.
Here's an experiment. Ask a group of guys, without their partners around, if casual, meaningless sex is possible and fun. Do they need to connect emotionally to a woman before they can enjoy sex? Can they have sex with someone and never think of it again, or not have it affect future relationships? I'd bet the majority of them will say casual sex is not only possible, but great fun. This no strings attached sex is normal to many men. That's not saying they think that's the best sex - most will admit it's better when it's with someone you love. But they'll still say it's enjoyable with someone you don't love too.
Ask the same questions of a group of women and you'll probably get a completely different response. For the majority of women, sex is very personal and emotional. They need to have a connection with their partner before they'll really enjoy sex.
While there's often an emotional connection between the cheater and their lover, I'd hazard a guess and say a good percentage of the time, the cheating "meant nothing" to the cheater. They didn't connect emotionally with the person they had sex with - they just had fun. The rush, the thrill, the sexual tension - all fun. They didn't mean to hurt their spouse, and likely hoped never to be caught, because they know their partner wouldn't feel it was just fun. Engaging in sexting, kissing, groping and sex with someone else, if your partner doesn't want you to, is a betrayal of trust. Even if you don't feel guilty about the act, because you know it was nothing to you, they won't feel that way.
If you're considering opening your marriage or trying swinging, both partners must have the same emotional response to the idea no strings attached sex. If she is convinced he's going to fall in love with every woman he has sex with, she's going to have a very hard time enjoying the experiment, and may end up feeling insecure, unloved, and heartbroken. If he feels upset about what she's doing, even if he's doing the same thing, that can be a big problem.
Ay, there's the rub. Jealousy.