Dude is a 30+ year old self-centered NEET virgin who is reincarnated as a magician in a fantasy setting. He then goes on to trying to have sex with 12 year old girls, but does it from a 10 year old body - he is actually 40+ years old but looks 2 years younger than his 12 year old victim, so that makes it A-OK in this sick fantasy.
With frequent passages of the MC getting hugely turned on and tempted sexually by children (we learn that this is normal and fun, apparently), we are also given some elf-like races that stop aging at 10 years old, so their women always look like lolitas and ppl don't have to feel guilty about all the sexual fantasies involving their child-like bodies...
So the MC is a pedophile who makes the dirty (worn) underwear he stole from the eternally loli (child-woman) his GOD - that's right, he openly practices a religion that deifies the idea of a world where children don't age and hence where you can have legal sex with a child's body.
Throughout the novels, anytime someone speaks against his 'religion' in the slightest way, the MC flips out and shows murderous intent towards them, making it clear that only so long as they are OK with his pedophilia is he willing to be a 'nice' (pedophile) buddy of theirs, but otherwise, then things are going to be very rough for his 'friends', and please remember that in this sick fantasy, the pedophile MC is orders of magnitude stronger than nearly everyone around him. Are the young readers supposed to learn life lessons from this?
Oh, and the old MC is surprised at the fact that he is not sexually attracted to his 6 yr old sisters, and ONLY because his sex drive strangely disconnects does he not to lust for them. It's not that he isn't trying to jump into his little sisters' pants because they are his sisters or because they are so young - noooo, it's JUST because he feels strangely not turned on by them, in spite of their looks! This was the very first time for him, implying that he had been turned on by all children regardless of family ties up 'till then!
The author, of course, decides to make one of the sisters lust after the MC, so we get countless scenes where, as a child and as his sister, she crassly puts the moves on him - the pedophile wish-fulfillment is enormous: readers get to picture how nice it would be to have a beautiful little child and family member lust after them constantly.