Your first bike should be one you know you'll outgrow. A 125cc is fine if you're practicing on city streets. Why he suggests brand-new, 500-plus-pound V-Twins up to 750cc is a mystery.
It's a real world issue. Ideally every new rider would start on a 125cc bike with a great big notification to the world that they are rookies...like a big "L" on their bike! And there would be horsepower restrictions! Wait a second...that's how the Red Coats do it! We fought a war so that anyone with a pulse can purchase and ride
any bike!
In the real world you work with disadvantages like:
1. Not a lot of 125s and 250s out there to buy. It's not a segment of the market that is pushed in the US. Most 125s an 250s in the US are called "step through scooters"--an honest to God motorcycle in that size is hard to find.
2. Most folks looking to ride have egos. Yes. It's true. That's a big part of why the small displacement segment of the market is pretty darn narrow. Nobody wants to start on a "baby" motorcycle even if old dudes like me find them to be a hoot.
So what do you do? In a perfect world everybody starts on a bike you can stall with both feet on the ground and then pick up and carry away. That is
never, ever going to happen. Blame Paul Revere and George Washington but in the states we don't roll that way and likely (you never know) we never will.
Rather than lecturing and going down a "safety fascist" course wannabe riders have to be reasoned with. A Honda Shadow is putting down 35 ponies. As is a Suzuki DR400. One's heavy, one's light. I'd pick the light one--except I can't pick for everyone they have to pick for themselves. Ninja 250s and 300s are around but picking up one on the cheap may be a challenge--I feel they and the CBR250 are extraordinarily wonderful first bikes. The issue is purchase cost and repair cost. (Besides what many man wants to learn on the "Baby Ninja"--a term of endearment that also works against the bike).
The real issue is that it's your first bike but newbs often treat it like the last. They want the dream and they want it now, hence the cornucopia of "Dude racks his Gixx 1K leaving dealer!" or "Dude wads his new Harley--on the ride home!" videos on YouTube.
If you stake out the extreme safety high ground (English style) then you end up alienating the moderate. Hey, there's always a kid with more money than brains who'll buy the new 1K 165hp bike, you're not reaching him. BUT you can reach out and get that willing to listen moderate segment. For me? The weight of a 650-750 v-twin is an issue but the horsepower balances things out. You could get to a bike that's 200lbs heavier and with 50 or more horses--but that doubles up the problem by making throttle mistakes more punishing.
It's a balancing act. Me? Buy used. Cheap. Light. Something that you can bend and not get bent about. Watch out for mad horsepower but be more wary of mad horsepower accentuated by big weight or hyper-reactive geometry. You're going to be entering a high risk environment so you best do it on a low risk ride or a ride you don't mind getting dinged up.