This seems obvious, but what does this mean in a relationship?

Men have no maternal instincts. This is nature, they can’t help this. They can’t become this.

Remember:

They don’t worry about our safety when there is no apparent harm.
They won’t call to see if you got home safely.
They won’t call you to tell you they got home safely.
They won’t come up with solutions if you’re sick.
They won’t notice if you seem down.
They typically won’t offer to hug you if you’re crying.
They have no sympathy if you are whining.