If the two of you were just roommates, this would make no sense.
But you aren't roommates, you are in such a committed relationship that you have lived together for 2 years. I am a big fan of the idea of having a test run before marriage. You learn a ton about people by living with them and seeing them all day every day. We also grow and mature over time, especially in our 20s. So you may genuinely love this woman, yet it may not turn out to be the person you want to live with for the rest of your life. So living together first just makes a ton of sense.
But if this is really a test run, and it has been going on for two years, then I think making some adjustments may make some sense even if the rationale wouldn't really apply to a roommate situation. If this is someone that you see life unfolding with - and living with her for two years suggests it is - then you need to view this as a partnership more than a roommate situation, and that partnership means that you are in this together. If she wants you to demonstrate that partnership-not-roommate mentality by only splitting the rent disproportionately, then I think you are actually coming out ahead in the short term. In the long term, if your relationship continues to evolve, then all of the money you both make will, either actually or practically, be pooled and you will make your financial decisions together.
This is the first step down this road. If you don't think you want to take those other steps, you have two choices: (i) cut bait now and save her the grief she will experience when she finds out what you already know (this relationship isn't going anywhere), or (ii) keep rolling with something that is fun, even if it isn't going anywhere, and pay a relatively small price of $70/month in doing so.
But don't delude yourself, you have been living together for 2 years, and she thinks this is going somewhere more permanent (reasonably so from an outsider's perspective). You refusing this will be about more to her than the $70.