きみ is one of those words that are common in fiction but rare in real life.
In fiction or lyrics, きみ is a common, androgynous, friendly and gentle pronoun, which is more casual than あなた but much less casual than おまえ/あんた/etc. For example it's used by:
- Someone who is a writing a letter to their partner or close friend (きみ is actually very common in songs for love and friendship, and its gender-neutrality is often very handy)
- A boy/girl who met a stranger of the same age
- A timid, gentle or leader-like character who doesn't like calling someone with おまえ/あんた (e.g. のび太 and 出木杉 in ドラえもん)
- A teacher/boss to their students/subordinates
In real life, however, きみ is rare anyway. A boss calling their subordinates is probably one of the cases きみ might be used in real life, although no one around me actually use it. Your partner might have felt your きみ sounded bossy and condescending. One real instance of きみ I definitely remember for now is...a stage performer who picked one of the audience using "きみ! (You!)". A few "委員長タイプ" students might be using きみ without being laughed at when I was a high school student, but I've almost forgotten.