There's a hundred of mods for it, and more than a hundred and half if you count both the original and Special edition. But it don't change the fact that it's not the first game of The Elder Scrolls series to have sex mods, nor the fact that The Elder Scrolls series isn't the first Bethesda series to have sex mods.
Something being relatively unknown to you doesn't mean that it was relatively unknown from the rest of the world.
There's more than thirty five thousands mods for Fallout New Vegas, more than thirty thousands for Oblivion, more than fifteen thousands for Fallout 3, and more than ten thousands for Morrowind, to just name Bethesda games.
NexusMods was created in April 2011, but come from TESSource, a site created around 2003 by and for the Morrowind modding community. A site that later became TESNexus, around 2007/2008, before it definitively cease to be dedicated to The Elder Scrolls series and started to act as a central point for all the modding communities.
I don't remember precisely, but within the few first months, it already hosted mods for more than twenty games. And, while the date of publication don't necessarily correspond, most of NexusMods' mods for Bethesda games released before 2011 come from the archives of the two original sites.
Then, two months after NexusMods final switch, loverslab was created by the part of the Fallout modding community that worked on sex mods, because NexusMods was more strict regarding the hosted content.
And all this happened before Skyrim release in November 2011...