Get nekkid, look at yourself in the mirror and answer your own questions, breh.
You getting a side piece because you want to continue having access to sex and a back up plan if your girl cheats on you or "Gets out of line", or you just want to fukk more than one chick?
Nah, for real. Be real with yourself, 'cause in my opinion having a contingency plan for something you could just scrap such as a relationship with a bytch that cheats on you is a bit counterproductive. 'cause you can't have no side piece and NOT smash, which in turn makes you a hypocrite 'cause you don't want your girl to cheat on you..but you putting yourself in a situation to cheat/chuck her ass in terms of maintaining said side piece relationship..by cheating on her.
I mean all people CAN cheat so no relationship is Ironclad, but you should focus more on finding out of this girl has the capabilities to be exclusive with you other than trying to find out HOW to get side chicks if you don't already know how to.
To answer the latter half of your question though, as far as side chicks go, you normally pull from the girls that know/like you already (You got some, right?

) and build on that. They know you got a girl, but she don't care, y'all just fukking and benefiting from each other from a sexual perspective. It's easier than meeting a new girl you don't know and having the stress of trying to keep her from finding out you have a relationship already as oppose to a girl who knows to keep her distance and creep.
Is it a good idea? Nope, especially if you out here asking questions which leads me to believe you might wake up a step slow and get your shyt exposed...but it's your life, breh.
