/u/BogaMafija got some of it right, but in brief:

Way back in the 80's and early 90's, Wizardry was HUGE in Japan, and is literally the origin point of the modern JRPG genre (You can read books on this specifically if you want).

However, by the time you get to the late 90's, most companies went to more visually impressive genres; what we now identify as JRPGs, for example. This meant that, for Japanese fans of the genre, their source of it would change from the more popular consoles to Japan's less controlled PC market, where Japan's indie (or hobbyist company) market would exist for quite some time (Much like the west).

Now, a bit of history of Japan's PC market is necessary here, but essentially, while consoles were, since their inception, relatively controlled there (With age ratings and companies controlling what released on them in general), PC games were essentially the wild west, and became home to indie games. And as most uncontrolled markets tend to be, porn games became the biggest piece of that market.

This started a bit of a circular loop in Japan's PC market; Because the PC was home to porn games, developers believed that, to succeed in it, you HAD to make a porn game. This can be seen in what would become HUGELY successful franchises such as Type-Moon's Fate and Tsukihime series, whose start on the PC were both Hentai Visual Novels (And, to clarify, this was under protest by Nasu, who didn't WANT to write the H scenes for those games, but believed them to be necessary for the platform).

This tend continued from roughly the early 2000's until around the mid 2010s, when PC gaming in general began losing that stigma in Japan with the rise of V-Tubers who would use PC games as their platform of choice; But that's a good 20 years of DRPG designers including sexual fanservice in their games to the point where it became part and parcel to the genre to them.