DH's biological father left him and his mother when he (DH), was a baby. He and my mother-in-law weren't married; he had a family (wife and kids), at home in Spain and returned to them. His wife acknowledged my husband and invited him over for occasional holidays when he was a child and adolescent - they were never especially close though. DH's father was 'troubled' - alcoholic, in and out of prison. He died ten years ago, when my DH was 25 - before we met. Since then DH isn't in regular contact with his Spanish relatives, though they email from time to time. DH went over there for his paternal grandmother's funeral four years ago, everyone was nice to him. His half-siblings sent us lovely gifts when the DCs were born and when we went to Spain a couple of years ago they came to where we were staying (a long way for them), to see us all.
Meanwhile, my mother-in-law met her current partner when DH was nine. They have been together ever since, living together, though never married. DH was sent to boarding school for most of his childhood as MIL worked abroad a lot. Her partner describes DH as his stepson, DH has never complained about this or asked him not to.
A few nights ago we were out with friends and I said "my father-in-law..." - meaning MIL's partner. DH did that jaw-clenching thing but I thought he was objecting to what I was saying (FIL did something stupid), sharing that with people. When we got home though he laid into me massively saying that his mother's partner wasn't his father and wasn't my father in law and I should never refer to him like that again.
I was a bit !!! - I said that as far as I was concerned he was my FIL, as he was MIL's longterm partner and had been around since way before I was. He sat on the top table at our wedding ffs! DCs consider him their granddad and vice versa. I am SURE I must have called him my FIL in passing though we usually use his first name in conversation.
DH and his mother's partner get on really well, no tensions at all. DH in no way lionises his bio father and in fact had a hard time when the DCs were born, especially when they got to the age he was when his bio father left.
So, what gives? I've tried to discuss it since but nothing doing. AIBU to think he's being a bit precious? There's no big (or small), family drama recently or anything.