Last name of father and mother, respectively.

  let motherLastName = "Carter Hughes"
  let fatherLastName = "Miller Thompson"
  let childLastName  = "Miller Carter"
  let childFullName  = "Jean Paul Miller Carter"

You might ask, —“Why does the father’s last name go first and the mother’s second?”— That’s an old tradition, and it can change whenever enough people in our society agree. As it stands, the father’s family name tends to persist down the family tree, while the mother’s family name often disappears in each generation.

Or so that is how it works in many countries around the world.

> What last name do your kids use?

In the country where he lives (Belgium), the parents get to decide which family name the kids get.

It hails from when family lines were important, and you can practically only have one line reflected in a name. Unsurprisingly, most societies considered the male's name to be the dominate lineage of interest, although that doesn't hold true 100% of the time.

By such a definition any tradition related to gender would be sexist. The tradition is that the wife will change her name. This tradition is why it makes up the majority of cases.

It is not sexist at all, let alone "obviously very sexist". Don't impute malicious motives to people like that, it's extremely rude.

I changed my name to my wife's name when we got married. Where I live, everyone can choose if they want to keep their name or change it to either ones. So its a free choice.

AND: Hope gmail will rollout this feature asap, so I can FINALLY adjust my email address too.