A young man is thirsty for older girls (onee-san) in isekai settings. After fapping 999 times, he’s teleported to a fantasy world to live out his fantasies.
This is a long running shounen manga with some ecchi elements and light hints at nudity. A spinoff manga, called Kenja no Mago SS, fully went to nude fanservice, but sadly only lasted 3 volumes.
If there was ever an award for the sexiest manga with the worst art, this manga would win it. I didn’t love it at first, but over the years it’s elevated itself all the way into the elite tier.