Siempre digo "People is". Supongo que es porque en español se dice "la gente es".

Tengo que acordarme de que es "people are".

How do you say "There are three people in the room"? (in Spanish )

Mediante una oración impersonal:

Hay tres personas en la habitación.

Si hay una persona, el verbo no varía.

En oraciones no impersonales, el verbo concuerda en número con el sujeto:

El regalo está en la caja.
Los regalos están en la caja.

OK - my example didn't help. But in English, the noun person has people as the plural:

There is one person in the room.
There are three people in the room.

This is not always the case, because there are set (usually legal) expressions, for example: (a coroners verdict)

Unlawfully killed by person or persons unknown

If you think of person as an irregular noun with people as plural, then you won't go wrong.

By the way - You said

The only thing that I'm able to deduce from that kind of thorny questions is that people who is in favour don't force anybody to do anything, whereas people who is against want to force everybody not to do anything.

Do you notice something odd here? You use a singular verb with people, and yet you continue (twice) with a plural verb (underlined). Do you see the inconsistency?

Yes, you're right, I didn't notice it.

A way to avoid this mess is to find another way of saying it.
For example: That's human nature.

Thanks, poli. But "people" is a very common word.

Of course people is a popular word, but there are may other choices. For example, at a slightly lower register you can use folks.

¿Tal vez te serviría una rima boba de lengua mixta?

¿La intentemos? Muy bien, ¡acompáñame!

¿Quiénes son la gente?,
se puede preguntar.
Pues, la gente es
the people are
un grupo a que
se le puede dar
saludos desde
near and far!