Have ye engaged in any anal play (on you) at all? I know when my guy mentioned it, I wasn't very gung-ho; not because I was "turned off" the idea, but because I was terrified about doing it wrong and looking foolish, or even injuring him! Even a couple of times playing with fingers or a dildo made me feel a lot more comfortable with how penetrating soemone else works, so I was much more keen (although still pretty scared).
If she's still not even interested in trying that level, try and see if she'll say why. Many people have concerns ranging from hygeine to "turning" their guy gay, and most of them can be addressed with some education and compromise.
If it's that she just genuinely isn't interested, maybe she'd be willing to try it as something for you, especially if that gives her free reign to ask for something selfish in return. Hopefully she'l be at least willing to try it once as a favour, and maybe get more into it from that, or at least be ok enough to do it now and then because she knows you'll like it. And if not, then at least ye tried.