It's just a really poorly written wish fulfilment show that exemplifies a lot of the creepier aspects of popular harem shows.
The hero finds himself in a fantasy world, where he is falsely accused of rape by the kingdom's princess. She doesn't really have any motivation for this other than just being an asshole. This incident causes him to hate and mistrust people, particularly women.
He then picks himself up a little girl as a slave (slavery is legal in this world). She "ages" (physically, not so much mentally) as she gains combat experience, so after she ages up physically he hooks up with her. She's still his slave. In fact she says she likes being his slave and that it's really cool and nice of him to enslave her. She gets mad at another character when he says he thinks slavery is wrong.
Then our hero picks up another slave, who is also a prepubescent little girl. She is mentally even younger than she appears (and she appears to be around 10) since she is, quite literally, a newborn baby. She is naked often.
As I understand it in the light novel he picks up yet another pre-pubescent girl who falls in love with him too.
It's just... gross.