It is basically a modification that adds mature content to the game such as sex and pregnancy.
for oblivion Downloadable sex mods
Note: This page contains links to nude body mods for Oblivion. Mods are user created files that add content to a game — usually for the better — that changes aspects of the game in many different ways. A Nude Mod for Oblivion, which brings some new armor and dialogues with them.
About Nude Mod for Oblivion
What are mods? Fortunately, this problem can be solved fairly easily by using the Import Graphics feature of Wrye Bash's Bashed Patch.