That sucks that you feel like you have to fake it with her. Maybe your relationship isn't that serious or to that point yet, but if you do plan on something long-term, this may not be the way to start your sex life out on the right foot.
I've been in the same situation, usually after a couple hours of continuous um... nookie... It shouldn't be a big deal to explain to your girlfriend that sometimes the rod misbehaves and is stubborn with a mind of its own.
And I'm sorry, but now I have this vision of the both of you faking it with each other... now wouldn't that be ironic..
