I don't think sex.com was an expected direct hit back then - and if I'm remembering correctly, even in the later 90s, you would have had to type http://www.sex.com to get it to even load right - there wasn't a ton of convenience and most browsers relied on a heavy set of built-in bookmarks to get people into portals and search engines.
That said, I believe it was totally true that they likely paid six or seven figures for a year or longer deal to guarantee top three for a ton of porn search terms. I assume that area was allowed to be seedy in order to mitigate risk of even allowing that content to be included in the first place, and companies didn't feel bad getting money from porn sites who had a lot to throw around.
For context, squatters would often land you on some bad sites and URI syntax was unforgiving, so it just isn’t something I remember most doing. There was a general desire for simple branding on the off chance you can get it typed in or printed on a business card or in an ad, but it was not the common entry point.
Also of note: UX expectations back then didn’t trust typing into a box and seeing any result first - it really was the solid ranking of Google results that made that UX something browser vendors adopted or more cynically, that Google pushed first with partnerships then with ownership of Chrome.
What changed is the cultural mainstream acceptance.. that something like PornHub can be a mainstream company. The market itself was alive and well pretty much the day after the movie camera left Menlo Park.
This wasn’t a technical requirement though, sex.com would’ve loaded just fine if it had an A record set.
This is back when you could fire up mosaic and read the what's new on the internet today :-)
People used to do that on the x.25 network to find interesting sites.