Order in some food, cuddle up on the couch, hell, light a few candles. For this to work, she's going to need to think this night revolves around her pleasure. The key to success lies in how you introduce the idea. Say the wrong thing and you'll be shutting down before you even hit the power button. Try to speak her language. I suggest something along the lines of:"I want to share my interests with you so that we can understand each other better." OR "My buddies want me to hang out with them but I'm feeling a little low tonight. Let's stay in and eat junk food and snuggle!"
2. A little booze never hurts-
Of course, I'm not promoting non-consensual gaming. However, a little social lubricant will help to get even the most reluctant girlfriend over her inhibitions and into whatever you two want to play. Just make sure she remembers the whole thing in the morning, else your sorry ass is back to level 1.