…that shouldn’t stop you. Here’s my history for perspective: started on a Ninja 250; got a GSX-R600 track bike; then got a DR-Z400sm city bike.
I wanted a GSX-R600 as my first bike. I REALLY did. They just looked so cool, and I’ve always been into sports cars, etc. One of my friends got a GSX-R600 as her first bike and loved it. And then I found this site. I ended up with a Ninja 250R and my god, was that the right decision. Having ridden the gixxer at 165mph at the track I can tell you with authority that, with the perspective I have now, I wouldn’t bother with a sportbike on the street. That’s why I bought the DR-Z (a supermoto bike). My slightly modded DR-Z still zips from 0-60 faster than a Porsche, but it tops out at 85 and has a friendly 400cc engine. It’s under 300lbs sopping wet, and it’s super easy to control, nimble, and stable at any legal speed. It can even haul passengers.
The honest truth is that, while they are gorgeous and sexy as all hell, true sportbikes aren’t really at home in the city. They’re really uncomfortable for long periods of time (read: over 30 minutes). They REALLY don’t like being ridden at low speeds. Any super-slow maneuvers like negotiating parking lots and u-turns are a royal pain in the ass. First gear is geared for 82mph. One second too much throttle can raise your speed 20mph. Too much front brake and the super-flickable steering geometry lets the front fork compress to the point of losing all rake (rake is the angle of the front forks; more rake means more stability and less ease in changing direction). At this point the bike because SUPER unstable (wants to just fall into a lean) and you can very easily endo the bike, tail over nose. Basically everything about it is geared for one very specific use, and one that doesn’t fit with city riding at all. Riding 2-up on a supersport is also incredibly uncomfortable and unsettling, both for you and your passenger.
The thing is, if what you really want is a city bike, I’d question whether or not you want a sportbike at all. Now the Ninja 250R is really just a standard in sportbike clothing, so I fully endorse that. It’s a fantastically fun bike…so much so that I still haven’t mustered up the guts to sell it. It will NOT however be a good bike for 2-up riding on the highway. You can do it, but I wouldn’t.
One more catch: you won’t be riding 2-up for a while (or at least, you shouldn’t be). The weight of a passenger, and the squeamishness of an inexperienced passenger in particular, can really change the way a motorcycle handles. All the things you struggle to do by yourself at the beginning will become difficult all over again when somebody gets on the back. For your safety, and that of your girlfriend, I would highly suggest waiting for at least several months (I would personally wait at least six).
I would also not get too caught up in your first bike. Most people in the motorcycling community get the itch for a second bike very quickly. They’ll either buy a second, or they’ll trade out their first. If your first bike is a Ninja 250, you can expect to sell it for almost what you paid. That gives you a lot of freedom to upgrade whenever you feel ready. I would suggest that route, but if you feel you must start a step up, I’d recommend the GS500 or some of the other low-displacement standards out there.
Anyway, sorry for blabbering, and good luck figuring things out.