i think i get a sense of what you believe cheating isn't, but will you define what you believe cheating is?
Cheating is when you get involved with another person on a romantic or complex level, mostly with the opposite sex, beyond just 2 same sex friends getting together, mutually playing with / satisfying each other, then going their separate ways until maybe a next time.
Cheating is denying your partner of your own sexual energy. Sexless unions make this a moot point. However, if your partner wants sex, but you've been too drained of your sexual energy after having had sex with someone else, then you're cheating your partner out of sexual energy. If you refuse sex with your partner, you are cheating your partner out of sex.
Cheating is having a relationship with someone that results in a transfer of wealth on some level to that person. You are cheating your partner out of wealth. This includes everything from a paid date out to buying something of considerable value like a car or a house. As this would constitute a relationship.
Cheating is when your partner really needs you for something, but instead you are off with someone else. You are cheating your partner out of time.
If you are in a commited relationship and your partner does not want to have sex with you, your partner is cheating you out of sex.
If you are in a heterosexual union, I don't consider engaging in a homosexual act on the side to be cheating. And in cases of mutual masturbation, it is rather a mere extension of self masturbation, projected onto another person. If you are playing with a cock or a pussy which your heterosexual partner doesn't even have, it's hard for me to see that as cheating. If you are willing to perform oral on your wife, how is sucking a cock on the side cheating, when you think about it.
If you believe (self) masturbation is cheating on your partner, you will never agree with any of the aforementioned.
Since it's not my situation, I should not weigh in on homosexual committed relationships, where a same sex partner has another same sex partner on the side. I suppose it would be cheating, especially if the partner were being cheated out of sexual energy, wealth, or time, as I've alluded to with a heterosexual relationship. But someone in a homosexual relationship would be more qualified to define what cheating is for them.
By now, I've come to expect a lot of disagreement on this stance, especially from folks who have an issue with insecurity, accepting this as reasonable. That's not to say you have to have insecurity issues to disagree, but chances are if you have insecurity issues, you are not likely to see this point of view.