(Most of the post below is copied from my earlier 'Finding Ur Examples' message.)
The Wikipedia article on visual novels currently states, "Some of Japan's earliest adventure games were erotic bishoujo games developed by Koei. In 1982, they released Night Life, the first commercial erotic computer game. It was a graphic adventure, with sexually explicit images."
Enix published The Portopia Serial Murder Case, as well as Lolita Syndrome. A couple years later, Square created an adult computer game called Alpha. Today, none of Square Enix or Koei's products are adults only.
The oldest game tagged as a dating simulation on vndb.org is a vintage computer game from Enix, called Tokyo Nanpa Street. I'm not sure if it has adult content, but the word 'nanpa' has been used in adult media.
As for raising sims with relationship elements, I'm not sure if anything was created before Gainax's 1991 version of Princess Maker. If I'm wrong, correct me.
Otome games as we know them began with the 1994 Super Famicom version of Koei's worksafe game Angelique.
According to a list on the Boys Love Games Headquarters message board (not very active lately), BL games began in 1999 with Joinac Gas's doujin game Graduation, and a commercial game called Boy x Boy Shiritsu Kouryougakuen Seishinryou.
Bara games began with the 2000 game Hotaru.
I'm not sure what the first game with *just* Girls Love content was.
Regarding visual novels published in English, three games which may qualify were released in 1994. These include two Sega CD games: The Space Adventure (based on Space Adventure Cobra), and Snatcher — as well as the computer game Knights of Xentar, which was a censored version of Dragon Knight 3.
Finally, visual novels originally written in English... I believe the first was Sinkha: Hyleyn. It was released simultaneously in English, French, and Italian in 2002. (I think that the trope maker for original English language visual novels was the 2003 freeware game Tales of Lemma.)