Does Magic Circle prevent unarmed melee attacks from crossing the circle's perimeter?

Question

The Magic Circle spell creates a circle that protects against certain types of creatures. The spell description states:

Choose one or more of the following types of creatures: celestials, elementals, fey, fiends, or undead. The circle affects a creature of the chosen type in the following ways:

• The creature can't willingly enter the cylinder by nonmagical means. If the creature tries to use teleportation or interplanar travel to do so, it must first succeed on a Charisma saving throw.

• The creature has disadvantage on attack rolls against targets within the cylinder.

Such a creature, from outside the cylinder, tries to make an unarmed melee attack against a creature fully within the cylinder. What happens?

1. They cannot do so, because part of their body entering the cylinder is treated the same as their whole body entering the cylinder.
2. They can do so, but have disadvantage. They are not trying to "enter" the cylinder, just briefly extending their body into it.

Answer 1

They would need to enter the cylinder, so they fail.

There's no exception for entering the circle only briefly, or only with part of your body. One of the implications of it being a cylinder is that it doesn't matter whether you're touching the ground inside the circle or just reaching across the border. The entire column of space is protected.

No, it doesn't matter whether the unarmed attack is magical.

The operative phrase is "enter the cylinder by nonmagical means". You can be a monk with magic fists or something, but they're still being moved by your muscles, not by magic.

Answer 2

Enter = Penetrate

The word enter can be defined in many ways. One way it is commonly defined is (Google):

penetrate (something)

And (dictionary.com):

penetrate or pierce

In this case the creature would be penetrating the Magic Circle. Since this is an act covered by the term enter it can't be done.