I am opposed to your proposal.
I am used to going home on foot.
Your friend is not supposed to encourage you to smoke.

Why do we need to use a passive voice in these sentence?
These aren't really passive voice. The verb 'be' can be used in many situations.
Not every occurrence of a form of to be followed by a past participle (am opposed, am used, is supposed) is a passive construction. None of these are passive voice constructions. is supposed is the only one that comes close to being a passive, but to be supposed to (do something) is better treated as an idiom.

The past participles of verbs are often used as adjectives. Many of these combinations form common idioms in English. In this case the construction is not passive.

He/She is depressed / annoyed / surprised / amazed / worried.

These seats are taken.
The door is closed.
The plates are broken.
These facts are well known.