You're constrained by the narrowest dimension(s) of an object, not the widest
You can put anything into a Bag of Holding that you can fit through the mouth of the Bag, so long as it isn't too big to fit in the Bag (64 cubic feet). A human is generally at least 5 feet tall, but in diameter they're usually much smaller. Only very heavy humans would be reasonably more than two feet 'wide' and therefore shouldn't have any issue fitting through the mouth of a Bag of Holding.
You should also keep in mind that the Bag is, well, a bag, it doesn't have a rigid shape. Therefore you could fit objects into the Bag that are wider and longer than two feet, so long as they're thin enough that you can pull the edges of the Bag out a bit.
If you've ever seen Mary Poppins, her bag has approximately the same mouth as a Bag of Holding (being generous with my approximation, but for the sake of the comparison I think it's solid) and she has no problem pulling very tall objects out of her bag. Likewise a (reasonably sized) ladder, coil of rope, human, or even a surfboard could reasonably fit into a Bag.
On overly-large humanoids
Almost completely unrelated, you may be misunderstanding the Size rules in 5e. It's important to remember that a Medium-sized creature does not fully occupy their 5'x5' space on the battlefield. Rather that space represents the area they have Control over, and the space they require in order to move freely. (See PHB p.191, "Space".) D&D Humans are shaped just like real Humans.