This is somewhat second hand but I worked with someone who claimed to know the person who owned sex.com (he was trying to buy it from him at the time, and I'm trying to be deliberately un-assertive) in the mid 90s (also fair warning, this was a long time ago and I probably misremember stuff) and said he paid a ton of money in bribes to the search engines/portals back then - which was what most people assumed, but those companies insisted wasn't true, so it fit a narrative people liked.
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.