Kissing someone else when you’re in an exclusive, monogamous, relationship is generally considered cheating unless you have different parameters in your relationship. If he made out with another girl, would you consider that him cheating on you?
If yes, then yes, messing around with other girls behind his back is cheating.
If you want it badly enough, talk to him about it. Ask him if he would be comfortable having an open relationship. You can set rules for it. Honestly, that rarely ever works out, but you can always try.
I, personally, would break up with him and explore my other needs. You’re young, you’re not married, there is no real commitment here. The fact that you’re on this blog asking me about it shows how much you really want to try something different. It also shows how things are just not really working in your current relationship. It doesn’t mean your boyfriend is a bad guy, a bad boyfriend, or that you don’t really care about him. It’s just not the right relationship for you if you’re seriously contemplating messing around with other people.
It’s scary to leave someone when you feel comfortable with them and you care about them. It’s scary risking being alone for a while. For most, it’s easier to be with someone than it is to be alone, even if it’s not exactly what you want in a relationship. That’s normal and completely understandable. However, when it comes down to it, you’re settling for less by staying with him. And you’re not doing him any favors by staying with him while thinking about other people.
Would you want him to stay with you if he was thinking about other girls and wanting to have sex with other people? No. Why would you want to be with someone unless they want you and only you? That’s the whole point of being together.
Gender is irrelevant. It’s the act itself that defines cheating.
My personal advice? End it on the best terms possible and do what you need to do for yourself. Do not mess around behind his back. Either break up or give him full disclosure on what you want. Because if you start doing stuff behind his back, lying to him, it’s a betrayal no matter what. And you can’t have a healthy relationship that way.