Interpretation of 思われる (spontaneous or passive?)

Question

My understanding is that 思われる, in addition to being the passive form of 思う, can also be used in the sense of "to spontaneously think; to appear".

北米では通常スカートは女性がはくものと思われている。
In North America a skirt is thought of as something a woman wears.

In which sense is 思われる being used here? Both seem reasonable:
Spontaneous: In North America, people spontaneously think a skirt is something a woman wears. Passive: A skirt is thought of by North Americans as something a woman wears.

I lean more towards the first one, since a passive sentence would normally mark the agent with に i.e.

スカートは北米人には女性がはくものと思われている。

Answer 1

From your last comment (emphasis added):

a verb that indicates what the speaker/writer feels spontaneously

This does not match the translation you give for "spontaneous:"

In North America, people spontaneously think a skirt is something a woman wears.

Do you see the difference? If 思われる were supposed to indicate the author's thoughts or feelings, a better translation would be

It seems that in North America skirts are usually worn by women.

However, with no other context, there is nothing to indicate that this is a personal observation of the author, so the translation

In North America a skirt is thought of as something a woman wears.

seems most appropriate to me.

Also note that it is quite common to leave out the agent in passive sentences. Just a few examples:

今日学校でいじめられた。
I got bullied at school today.

政府は倒された。
The government was overthrown.

彼は殺された。
He was killed.

Answer 2

The 自発 (spontaneous) 思われる is a static verb (like いる and ある) and cannot take the ている form. So the 思われている in the example is definitely a passive.

The passive 思われる can take the 思われている form based on tense/aspect.