I've been thinking about this a lot. I think creativity peaked in the 1970s to around the late 80s, VERY early 1990s (like 1992 or 1993). Like you could do whatever and people didn't give a shit and enjoyed it. People were going all out. So what about fan service? Who gives a shit? But now, everything is so sensitive and if you so too much cleavage people get the vapors. This secular Victorianism is getting really fucking old.
I wouldn't necessarily say it was a ground breaking creative period in the medium, rather that it ended up being the only periods in the industry where there was any major innovations that ended up becoming the standard tropes.
The 70s had an expansion of manga and anime as a popular medium outside of the fringes that it had existed in during the early postwar and rebuilding period. The first major influences on the industry for better or for worse was Star Wars and the sudden fascination with space themes, and this was also reflected in western animation and science fiction as well.
That said for every ground breaking film or genre creation, there were just as many sub-par clones and derivative works trying to cash in on the animation booms of those periods. Undoubtedly some of the best and most influential works have come out of that period, but we tend to have a Pareto Principle view of that time period because we only really treasure the anime that has stood the test of time. The other 80 percent of crap from the period has fallen away. (Or in the North American anime boom, badly dubbed by Manga and sold over priced at HMV)
Now in regards to sexualization in anime for that period, I think it generally has three influences.
1) The lack of a viable Hentai oriented industry.
A lot of small film companies were trying to produce anime in a small market, even smaller in this market was the specialized adult market, which was always a second tier industry in terms of animations, preferring to focus on games and manga during the industry development period. As a result you had a lot of salacious stuff being put into anime during the period, because sex sells and they knew there was a bigger market return on films that had some form of nudity, fan service, or sexuality implied or explicit.
2) A more adult/young adult based core audience.
The early anime audiences were usually male based and in the older teenager/adult range. While a lot of anime was produced for children, and the TV shows of the time reflect this. (The original devil man syndication is an excellent example) Movies and more expressly when the market opened up to OVA's had access to more mature audiences, and you can see a clear delineation in the industry between TV and OVA, a classical example from the early 90's period is Bakurestu Hunter, the series which include female characters in semi-bondage gear was fairly avantgarde even for that later period of industry development, however it is in the OVA that you have extended sequences of characters bathing, multiple nude scenes, cruder jokes, and the original costumes from the manga.
3) Cultural restrictions on what could be shown in actual comparable cinema of the period.
This is not so much a 90's anime issue, as it would begin to even out with the OVA and then become more conservative again in the 2000's, when the industry started to implode. (Too much of the same derivative crap, lower product budgets, lower market returns, creative bankruptcy)
However for a brief period in Japan Anime was an industry leader in opening up new ideas to the Japanese public. The Japanese are conservative by nature, and it was Anime that started loosening the old conservative ideas that dictated the cinema industry and television creative output for the first time since the pre-war period.
Ironically the Tojo wartime government and postwar rebuilding period made Japan more conservative. General MacArthur's edict of censorship still exist today in law, which is why pixels and censorship is still used for genitals.
The sexualization and popularizing of tentacles in anime also grew up around this form of censorship, as it was allowable by censors to show implied penetration, and tentacles since they did not equate to sexual appendages served as proxies.
Sexually-explicit horror anime peaked in the late 1980s and early 1990s, during the OVA boom when most mature anime went straight-to-video where animation studios didn't have the sorts of restrictions that broadcast television would impose on them. The move away from OVAs towards late night TV for mature anime starting in the late 1990s is a huge reason why that subgenre has largely gone extinct.
I'd say it was partly that, but as well as the development of the adult market as well very much removed the need for more sexually explicit series for the mainstream market.
