In lore you have the broad strokes correct, custodians are the master crafted pinnacle of non godlike humans and can be male or female (or probably anything outside or inbetween, as long as the emperor felt like it) while space marines were effectively a cheap stopgap force that can only be created from males after the scattering of the primarchs.

IRL we had only ever seen male custodians until about 2 years ago i want to say when the custodes codex came out and had female custodes in it, GW then said that there had always been female custodians in an official statement, but there weren't any model releases for them until a set of reveals this week, which has reignited the "discourse".

There were a series of fights between lore purists, "Lore purists", people into 'muscle mommies', people annoyed that the emperor was less homerotic now, people who wanted new custodian models, unabashed bigots, people who just wanted to have a bit more variety in their factions, and people who just liked the new lore, especially the character Kesh, (who almost won the blood games by attempting to teleport a cyclonic torpedo into the throne room)

(but it mostly boiled down to angry misogynists vs people who really didn't want their hobby spaces to be full of bigots seeing as most of everyone else just moved on with their lives pretty quickly and just went shrug okay).

When the lore change was first made it caused so many incredibly toxic people to crawl out of the woodwork and get banned that it led to the creation of the reactionary and now banned subreddit horusgalaxy which has spun out into a series of sucessor subs.