Learn Finnish: Onko ne vielä kihloissa?

In spoken Finnish (puhekieli), the pronoun ne is used for both things and people. When ne is the subject in spoken Finnish, the verb is always conjugated in the third-person singular (the se or hän form). So instead of the standard Ovatko he, people say Onko ne.

Is it rude or objectifying to refer to people as ne (those/them)?

Not at all in everyday spoken Finnish. While se (it) and ne (those) are strictly for animals and things in standard written Finnish, they are the completely normal and polite way to refer to people in casual speech. Using the formal he in a relaxed chat would actually sound unnaturally stiff.

How would you write this sentence in formal written Finnish (kirjakieli)?

In standard written Finnish, you must use the pronoun meant for people (he) and the matching third-person plural verb form (ovat). The sentence would become: Ovatko he vielä kihloissa?

Why is the word kihloissa in the plural inessive (-issa) case?

Kihloissa is the fixed expression for being engaged. It comes from the word kihla, which historically meant a pledge or a betrothal gift, and nowadays refers to an engagement ring. Because the ending -issa means in, being engaged in Finnish literally translates to being in rings or in pledges.