Experimentation to satisfy a curiosity or a one-off due to not having access to a woman at the moment is one thing. But continuous, regular sex with other men is another. A guy receiving regular BJs from other men and still being both attracted to women and having steady sex with them is bisexual no matter what else they chose to call themselves.
A man who is truly straight and not getting what he desires sexually with his female partner would not seek out other men, but other women if he decides to go the cheating route. Indeed, if he was straight he would not be capable of sustained arousal to complete a sexual act with another man. Those that have regular sex with men and still claim they are completely straight are in denial because of the perceived gay or bi stigma. They will twist themselves into pretzel logic that they are not really bi because it's only another guy's mouth or they don't suck dick themselves or they still have sex with their female partner, etc.
The gay or other bi men that service them will also not see them as really bi either, because it fits in with their fantasy obsession with having sex with straight men and that they are someone different or better.
They should just own up to it and don't list themselves as 100% (or even 99%) straight if they are regularly getting sucked on or ass pounded by other guys. But the only really morally indefensible issue in all this is the fact these guys are doing this behind a female partner's back because she isn't giving them what they want. Trying to use the excuse that they are not being unfaithful because it's not another woman is another way they are in denial. If the man is cheating on his wife or girlfriend, the gender of the other party is irrelevant and he is garbage.
