Yes, by being honest with your "main chick" about whats going on. Having sex with lots of other girls while you have a girlfriend does not make you the man, it makes you a liar.
Essentially the situation you want is to have a nice relationship with the first girl and just call the other ones up to **** whenever you feel like a change. You want your cake and you want to eat it too. I've been there and done that when I was younger. And yes, it is very easily done. Have sex with other girls, guard your cell phone/email/instant messenger with your life, and perpetually spit bull **** at the girl that you care about enough to call your "main chick". Or you could de-stress and think about the situation for a minute.
What do you want? The answer apparently is sex with a lot of girls. What is the obstacle? In your mind, the obstacle to this is having a girlfriend. Simple solution? Eliminate your girlfriend as an obstacle by either breaking up with her, going on a break, or simply being honest with her about why you want to have sex with other girls. If you want to have sex with other girls and plan on actively pursuing it, what is the point of lying to yourself and other people that you care about by being in a committed relationship?
I am willing to say that you are probably 16-19 years old, with your main girl falling into the same age group. You guys are probably each others first "serious" boyfriend and girlfriend. What are the chances you are going to marry this girl? Not very good, so what is the point in making yourself unhappy by denying yourself the freedom to chase other skirts and making her unhappy by your dilly-dallying? The clear answer is that there is no reason for this. Adopt honesty as your main policy and a lot of the headaches that surround women will disappear.