Lenny was about 20 years older than me. Neither one of us was wired for sexual jealousy, and we each occasionally had sex with other people. Extramarital sex was never a huge thing with us, but if and when we did, the rules were that we would tell each other about it, and that we would (of course) play safe.
The BIG rule was that we remain committed to each other in time and attention. We spent most of our free time together, and that was the most important thing.
Neither one of us was out running around on the other, having secret affairs, or letting our partnership deteriorate. That worked well for us, though I know it wouldn’t for some.
To each their own!