There is nothing "immature" about a mountain bike. I am by no means dissing your taste, just sharing an observation: people's tastes in bikes change after they've actually done a decent amount biked for a while. The god-awful crap that department stores push as "bikes" is an illustration. Millions look at them and think "hey, what a sleek-looking bike!" Most non-cyclists don't understand why you would pay 800 dollars for a bike-shop bike, when you could pick up a nice-looking machine from Walmart for $129.99. And I look at Walmart bikes and want to vomit.
And it's not just that I theoretically know they are bad bikes. It's a gut reaction by now. They look plain ugly.
That said, one thing is certain: you gotta love your ride. In the end of the day, I would never encourage anyone to buy what I think is a great bike if they're lukewarm about the looks. It counts. But I've seen so many people pick a bike just based on looks and get completely disappointed because that's not at all what they really needed. I've seen women pick some beach cruiser type bikes because in their minds that's what women's bikes must look like, and then never ride them because they're slow and heavy and clunky machines. (That's a particular pet peeve of mine... women's bikes are NOT the same thing as bikes with step-through frames! *sigh*) I've seen people go and buy a bike with the biggest saddle and upright geometry because it looks "comfy", and then regret it because they find out the saddle chafes their inner thigh and the upright geometry actually hurts their back and makes the kind of cycling they're hoping to do completely inefficient. I've seen people go and drop a crapload of money on a bike a size too small because the correct size looked "intimidating". I've seen people buy hybrid bikes when a touring/cyclocross/road bike would have suited their needs better because they were scared of the drop handlebars.....
The list goes on. So please, don't go by looks alone. Two pieces of advice:
1) Find a way to at least test-ride a few different kinds of bikes. Even better, rent different kinds of bikes for a day or two, if you can afford it, or borrow from friends... That'll at least give you an idea of how different kinds of bikes ride.
2) Don't spend a fortune on your first bike. You will most likely buy something that isn't quite right for you, just because of inexperience and the overwhelming number of options. Go on craigslist (rigid mtb would be my first choice too, but a hybrid might be fine as well). Buy a used bike, ride it for a year. Do try other bikes throughout the year if you get a chance. By the end of the year you'll have a better idea of what you need, and what kind of a bike would provide that.
3) Learn to adjust the bike for best fit (saddle height, forward/aft position). And do not judge the comfort of the bike by how comfortable the saddle feels. Saddles are easily swappable. An otherwise ideal bike for you might feel like the worst bike in the world if it doesn't have a saddle that fits. But that can be fixed. What can't be fixed is the basic frame geometry and quality of the components.
Here are a few bikes off Toronto craigslist I'd consider if I were buying one right now:
Steve Bauer 18 speed
Specialized mountain bike - with 17" frame (Specialized Hard Rock... nostalgia!)
Norco bicycle (my experience with old Norcos is that they're tank-like... but I'd still take a look)
Miele hybrid bike (oooh, my kind of geometry!)
Miele Cicli chrome-moly commuter bike
Obviously, it needs to fit and to actually be in good condition. Good luck!