I'm a father whose daughter has my last name instead of my partners.
We were together less than a year when our daughter was born (Very happy accident). We'd both said we were uninterested in marriage, and weren't even living together at that point.
I just assumed that she'd have her mothers name, to the point that when we finally decided it would sound stupid with her last name (think Lois Lane, Lana Lang, Pepper Potts type alliteration), at which point partner said she wanted her to have my surname. I was surprised, we were both being very sensible about the fact that this was a very short relationship that'd had an absolute bomb dropped on it, and we were under no illusions that we'd definitely still be together in 3 months time.
Partner said that if we split up, then daughter would be spending bulk of time with her (which I thought was fair enough with a young child), so she wanted me to have the surname bond with her. I didn't really have an argument against it, so thats what happened.
15 years later, we're still together, still not married, daughter still has my surname and not her Mums. We have the "marriage" conversation once a year, its become a tradition at this point, and have never decided its worth changing the status quo.
Purely from a logistics point of view, I'm so glad that daughter has my surname. As a father you get questioned so much more about your relationship with a child.
For instance, me and daughter went to the Harry Potter studio tour last year. Partner couldn't have been less interested so just the two of us went. Stayed in a hotel the night before, and even with the same surname on the booking, I still got questioned on my relationship to my daughter, and receptionist confirmed with daughter too.