This is an article rather than an official study but makes some good points:
Especially in this paragraph:
So, try not to give the impression to your female partner that she ‘ought’ to be able to climax through intercourse alone and that that is what you think of as ‘proper sex’. The sex menu can be a varied one.
I know that the experience of one person may not mean much or go that far is convincing her that she's normal, but I also have a lot of trouble. Even with manual and/or oral stimulation, less than 10% of my partners have been able to make me come, and until my current boyfriend, I was pretty sure it was never going to happen. I've definitely never come from sex alone - it would be an impossibility. It think this is the case for a lot of women, from my own experience talking to people about it, it's quite rare for a woman to be able to come without any clitoral stimulation.