Only because that's what we're expected to do in this society now. We are expected to have a partner. We are mislead to the point where we see a need to have a partner.
It's how our society works, not human nature or any real means to having a positive lifestyle. I mean, humans aren't even on the list of monogamous species.
That said, if your priorities are set then you need to tell her what they are. She's just another pretty girl and if she can't accept your passions then you can't really have a working relationship. If she's like that then she is an ignorant, conceited bitch.
InB4 "You have never had a girlfriend" (I have. I have rejected several ladies in the last few months on the basis of my cynical but more realistic approach to relationships.)