John Kowalski
Banned
It matters because it's only being called sexist because it's regarding a woman taking a man's last name. If in a gay couple, one wants the other guy to take his last name, is that still sexist? No it's not. The majority of people like the tradition of one last name and they aren't likely doing to be sexist. It shouldn't be assumed that it's under sexist reasoning.
That still has no bearing on whether the custom of a woman taking a man's name is sexist or not, of course sexism only exists when there's sex specific discrimination happening. You're making a ridiculous comparison, frankly, and i apologize for the bluntness but that's like saying that a white slaver only having white slaves means that black slavery shouldn't be assumed to be racist, a point whose sheer inadequacy should be fairly obvious, given the historical context of slavery in the US.
Otherwise, what's the distinction between not being sexist and participating in a sexist custom, and participating while also being sexist? At the end of the day both are acting in ways that are discriminating, the same way that it doesn't matter if you're not a racist and vote for Trump, voting for Trump is actively engaging in a choice that will have racist repercussions.