100% agree with the above poster.
But there are a lot of reasons a girl may only want to give a bj too. Has rationalized she's not a slut if she doesn't go "all the way," is on her period, has a bit of funk going on down there - whether std or bad smell (i.e. she might be doing you a favor). Maybe she even made some dumb agreement with friends that they wouldn't sleep with anyone new for a while...who knows.
All that said, accept the head and call it a night - I don't see what the problem here is. haha
For the record, though, my usual line for something like this is "I don't want you to do anything you're not comfortable with." Then back off a little and try again later anyways.
Another option: a friend taught me back in high school that if a girl is ever resisting sex, go down on her and she'll get horny enough to throw caution to the wind. I can only think of one time I ever regretted taking this advice. This worked for me more times than I can remember through my 20s.
She might not let you (I think most women know they can't control themselves after getting eaten out). And it can even backfire, but that's really rare in my experience.