Idiots! Saying you can't ride - HA! Show them!
First check: http://cycle-ergo.com/ You can barely stand on the Ninja, you will be tippy toes, difficult to hold it up at a stop. You will find that a cruiser, with it's low saddle, would be far better -- you can hold the bike at a stop with your feet solidly flat on ground. Strength is not needed -- technique will suffice. You can easily put a dropped bike back on it's wheels.
But it is easier to ride a lighter bike, which is why I ride my big single. Normally people will suggest you start with one of the 250cc class cruisers, the Honda Rebel, Yamaha V-star, Suzuki GZ250 -- all easy, all low, all at about 330 pounds wet.
But since all your guy friends ride, I would suggest my bike, the Suzuki LS650, Savage or S40 depending on the year. It is just 50 pounds heavier than the 250's, but with a 652cc single cylinder engine. It is not a fast motorcycle, only 30 hp, top speed about 85 mph -- fast enough on any legal street and 10-15 mph more. But it has torque, lots of low speed torque. Torque is what accelerates you at the green light. When the signal changes you will be across the intersection than any multi upto 1000cc. Tell you too small?! THEY are too big and heavy and SLOW (off the line.)