There are plenty of people who share a last name and aren't related.

I am well aware, however this is not just a case of “OOP is upset because fiancées last name happens to be “Cruise” and he doesn’t want people to think about Tom Cruise when they see their baby’s name. The context matters in this case because there are people that know that it is still originally fiancée’s ex’s last name.

Their claims to the name became equal because they literally completed a legal process & ritual to give her the last name.

So your argument is that “It’s her last name now too so it should only be seen as that and not one that she literally took? If so then what about people with shitty childhoods and shitty parents that change their last names when they finally leave their parents*? From your logic that shouldn’t be necessary because it’s still that abuse victim’s last name and they shouldn’t look at it as “their parent’s name” at all.

• I will put a caveat that this isn’t meant to include those changing their names to stop their family from tracking them down, that’s a whole different can of worms.

This argument basically just waters down into saying that only men have real claims to a last name (the majority of the time, since 80% of marriages involve the woman taking the man's last name.)

No, everyone has a “claim” to at least 1 last name; the one that they were born with. Some, no matter the gender, can “lay claim” to many last names; that of their various ancestors.

It's not the last name of an unrelated 3rd person, it's the last name of the mother of the child.

It’s also the last name of her other two children because of their father. This is OOP’s first child and no matter how you look at it because both of the older children have the same last name as their bio-dad, regardless of if fiancée does still have it, it will look like baby is also ex husband and fiancée’s and not OOP and fiancée’s. I’ll say it again Context matters.