Well, Bro, it depends. Your post history sends out clear "I don't understand consent" vibes, but previous posters have already told you that.
But let's forget about you for the moment and talk about how we know if a relationship is or is not exclusive.
If you're in a genuinely open relationship where both parties are happy to have sex, romance, life commitment, etc. with others, then have at it - although of course there's no one size fits all so there are mutually agreed-upon boundaries and if you violate or lie, there's no excuse and your partner will dump you (or vice versa).
If you have some kind of explicet agreement with a partner about sexual fidelity then yes, it's "cheating" to break that agreement.
If you have an agreement with some kind of partner that you're "together" in some way but not sexually (or emotionally, romantically, or whatever) exclusive, then only the two of you know when or if someone has violated the line on that agreement.
If you're just casually having sex with someone and you happen to also be having it with other people, then use your common sense: does the other person know what you're doing? You're not required to tell all, but pick up a culturally specific clue: maybe in any ongoing relationship, it's good to talk and agree about these things.
If your partner's a woman and you're a man, there are a whole host of extra things that come into play like risk of pregnancy, variable vulnerability to sexually transmitted diseases, etc. that everyone involved should be considering. I'm sure you knew that, though.