Don't. Seriously, just don't do it.
I know it's easier said than done, but no matter how much you care about him, how secretive you two will be, or how good the sex is, you will regret it.
You'll sit in your room wondering if there's a reason why he hasn't come to hang out with you yet today. You'll wonder if it's something you did or if he's just busy with his actual girlfriend. You'll wonder if he's done with you or if he's just not interested right now. You'll think to yourself that maybe you should hold out on him the next time he asks, just so he can see what it feels like.
But despite this, you'll still give in the next time he comes looking for sex. You might do it because you feel lonely, because you just really enjoy it, or because you don't want him to think that you've developed feelings and you might get hurt.
And that's the other thing - you can't let him know if you've developed feelings. Because that will be part of the agreement, since he's not planning on leaving his girlfriend anytime soon. If you develop feelings, you'll either decide that it's time to cut things off, or you'll keep going even though you might get hurt in the process.
And you will get hurt. You'll see him with his girlfriend and know that the two of them are happy. You'll see them kiss, not having to hide it, and wish that it was you instead. You'll see everything that happens in their relationship and you'll wish that it could be you instead.
And you'll try to tell yourself that she's a good person and deserves him. You'll also get annoyed by her for no reason. You'll realize later that the only reason you hate her is because you wish you could be her. You'll want the relationship they have and you'll spend all of your time focusing on that impossibility.
You'll let other opportunities pass you by. You'll wait around for him every day, hoping you'll get to see him. And when you do hang out, it's only ever for sex. This will be the worst feeling, especially if you started out as friends and then became friends with benefits. Because you'll feel like he only ever wants to see you when he's looking for sex. And that will make you feel like that's all you're good for to him. Maybe you start seeing someone and you worry that you'll have to cut things off so that no one figures anything out. You'll hide your feelings until it destroys you.
All of these problems and you're not even the only one at risk here. Think about her. Think about the relationship they have. Don't picture it for the sake of seeing yourself there instead of her. Picture it for the sake of imagining what it would be like if she found out. All of that worthlessness that you'll feel from being the side chick? Multiply it by a thousand. Because that's what she'll feel like when she finds out her boyfriend has been sleeping with someone else. She'll feel like she isn't good enough because he turns to someone else for sex. You'll feel like you aren't good enough because he turns to someone else for a relationship.
Even if you consider all of this and still think it will be worth it, please just take my word for it when I say that it's not. It is not worth it. You could screw someone else over, and you will definitely screw yourself over. There is no happy ending, no matter how you imagine things playing out. You may think that he cares about you, and he might actually, but I guarantee that you will ruin friendships and relationships if you decide to become the side chick.
You may think that having sex will make you feel closer to someone, but it will make you the loneliest you've ever felt if you're not allowed to have an emotional attachment.
So for your sake, for her sake, and for the sake of anyone who it could possibly affect, please do not become the side chick. You are worth more than that, and you'll find someone who can make you feel the way you think he will. They'll be everything this guy is and more, because they'll actually be able to commit to you.
I'm telling you all of this from experience, so please take my word for it before you decide to test it out for yourself.