Mato Seihei no Slave

The demon-filled dimension known as "Mato" has opened to threaten humanity, and a type of "peach" tree that grows there yields fruit that confers special powers to those who eat it, giving humanity a fighting chance, but this only works for women. Subsequently, as protectors of humanity, women rise in social status while men are relegated to second-class citizens scraping for basic occupation and acknowledgment.

Cue Yuuki Wakura, a domestically crafty high school senior whose sister was taken during a Mato event five years before. While lamenting the hardships of the life he'll soon have to face, an event opens before him trapping him in the Mato. He is immediately rescued from demons by Captain Kyouka Uzen of the Anti-Demon Corps, who then confers to him powers as her "slave" which turns him into a mighty beast mount that mows down the demons.

However, there is a surprising price to pay for being her "slave"... and it's not him who has to pay it.

Original manga online