You have to assess your problem. This is happening because your gf is not afraid of you. She is not afraid of your anger, and she's not afraid of you leaving her.
Why?
1) Because you're anger really has no consequence. It seems like you're more afraid to lose her than her losing you.
2) She's not afraid of you leaving her because she already has guys (at least 1 guy you know of) that she is keeping at arms length.
Solutions?
I personally would take a more assertive action but I don't really see you having the cajones to do it. I would just say fuck it to chivalry (since I've always had the opinion that chivalry is privilege, not an obligation). I would make it clear what my boundaries are, and they aren't just limited to sexual actions. There are many steps preceding sex that I already consider as crossing my line.
2) and this is seriously what I would do. This is a lost cause until you actually do something dramatic and drastic.
I would cheat. No joke. And wouldn't hesitate.
The thing is, to her, you're probably just an object, a commodity, something convenient for her at the time being. Eventually she's going to learn one day that you can't treat people as commodity. But in my life, this is why I've come to terms when it comes to cheating:
It's better for her to learn this lesson at her expense rather than yours (I mean why not? its her lesson, not yours). I don't think you guys are getting married at this point so if it were to end, at least let it be on your terms and in your favour rather than in her terms and in her favour (which is what she's building up to IMO).
Yes, you're going to be a bad guy for a while but seriously, bad guys have all the fun
Seriously, even if the thing with the new guy were to fail and amount to nothing. I would seriously have more pride than to be some girls "safety net."