Apparently I was "being a dick".Disclaimer: I sleep with this person.
I was going to say- that person shouldn't receive your reference. But now that I know it was your girlfriend, it puts another spin on it and I am wondering what the tone of the conversation was. But it's a little odd to use your partner as a reference anyway; it should be a professional contact.
If my wife asked me to be a reference, my response would be "sure, what for?". So, I think asking what you're being a reference for is perfectly acceptable just so you know how to answer should the person seeking out the reference start asking you questions. Last thing you want is to be completely caught off guard and have someone calling you and start asking questions about the person and you're response is "wait, what's this all about?" to the caller. Then, that reflects somewhat poorly on the original person that asked you to be a reference.
Now, if your significant other said "hey, can you be a reference for me" and your response was "what for" (coming off more like "why should I do this for you?") without also saying yes, then yeah, I can get why their reaction would be to think that you're being a dick. But still, realistically, they should be telling you what you're going to be a reference for so you have all of the information.
So what was the reference for, if not for a job application?And when you asked why this angered him or her so much, what did they answer?