Shinsou Parallel Paradox -Heikou Sekai Risoukyou- by WitchFlame - Adult Sex Game

The protagonist is somebody who is capable of travelling between parallel worlds. However, because he's been using this power for as long as he can remember, he doesn't think of his ability as something special.

But one day, he realises that he doesn't have counterparts in each of the parallel worlds. Believing that the worlds revolve around him, he gains a superiority complex and decides to go and satisfy his desires.