I agree with the general trend: This could easily be a relationship killer, and if it's bad now, it'll only get worse.
If you really, really want this relationship to last, you need to talk this out OR work around to that by talking about sex in general and, "Hey, can we try role playing?" If she'll go for that, it's a start.
I've known several girls and a couple of older women who wouldn't do it. After talking it out, or role playing a bit, if she still wouldn't, we parted. My best experience was a girl who was doing me on the sly; she finally agreed to try it and discovered that she loved it. Too bad she was married, but her husband is a lucky man!