It's only cheating if you're breaking a rule to not sleep with other people. Swingers aren't cheating (assuming they're respecting any agreed upon boundary) when they're having an orgy for example.
A bi man with a wife and a family could very well be having sexual encounters with men once in a while with the agreement of his wife (who could well have her own stuff going on too) and it could be a very healthy thing in what is a fulfilling relationship for both parties.
You might be right morally, but in a lot of countries it is still the case that 'not having sex with other people' is a compulsory clause of the marriage contract. So legally it's a little bit murkier.
People have been advocating for marriage contracts that are a bit like the CC license, where you can pick the parts that you agree on.
Marriage is on its way out in many places. In my region of Canada (Quebec), less than 50% of couples with kids are married. That number keeps going down every year.
(There are some automatic protections in the event of a separation.)
A bi man with a wife and a family could very well be having sexual encounters with men once in a while with the agreement of his wife (who could well have her own stuff going on too) and it could be a very healthy thing in what is a fulfilling relationship for both parties.