I was 19 at the time, actually. No she's not my first girlfriend, and no I'm not going to "slow down".
I regret making this topic as I didn't realize moral GAF would be running in. Once again, I'm not looking for advice, as the details of my relationship were not entirely written in my opening post. Just wondering what others would do in the situation. Now I know :lol
By the way, the girl that she wants to have sex with is a friend of mine, and we've been together (the three of us) recently, so I am completely trusting everyone at every step of the situation here. I see nothing different between those who would be okay watching and me not being there to watch but being aware of it.