6 jar of sigmoid neuron (I): Sigmoid Model part 2

In this video, we saw that y = 1/(1 + exp(-(w1x1 + w2x2 + b)). Generally, it is recommended to use sigmoid for only two inputs.

We can also use it for 1 input models in which case well, obtain a 2D plot instead of a 3D plot as in the video
Why this option is wrong! Explanation.