I remember asking myself that question when I got into my first-ever relationship, my freshman year of college. I honestly had no idea what a girlfriend's role in my life was supposed to actually be; namely, her "duties" to the relationship. I just knew my own "duties": regularly calling her (texting didn't exist back then) to see how she's doing, making her feel special and cared for, paying for her meals, helping her with coursework, spending as much time with her as possible, creating romantic experiences for her, and protecting her from dangers (which never came up, and I'd probably fail at it if they did).
The one thing I strongly expected from her was being by date for events and one-on-one outings. (That was the definition of a girlfriend in my mind back then.) She did a very poor "job" at it, and I viewed that as the norm, blaming myself for "not being romantic enough". I didn't expect sex; due to the messages I picked up from teenage sitcoms, expecting it was "wrong", so I didn't even dare mention anything sexual. I also didn't see breaking up with her as an option, because I "knew" that I'd never find another girlfriend again. I was putting in a lot, and getting very little in return. But even then, I was very much thankful for the opportunity to be in a relationship.
I think that shaped my view toward my future relationships for years to come. My relationships in 2007 and 2009 got sexual very fast and had no settling-down expectations, but ended equally fast too. My next relationship, in 2011 thru 2013, had another problem: my girlfriend kept pushing me to settle down with her, and treated me poorly. Which affects me to this day: I'm afraid of women forcing me to settle down, and treating me poorly as a way of achieving that goal. So now, I prefer platonic female friends for fun and companionship (at least the kind impossible with a man, like dancing), and escorts for sex.