The Main Character (MC) wants to join the demon army and he fought one of the demon generals as a test to see if his worthy to join.
After they fought, she got defeated. The demon general which is a type of beast-girl fell in love with the MC and wanted to marry him because she likes strong men.
I don't remember if the manga is R-18 but it's not censored.
I read it 5 years ago, it had a few chapters but I think it has way more now after I forgot about it
It's a typical overpowered MC manga, but it's pretty interesting
I've tried to see if " i'm quitting heroing " is the manga i've been looking for but it isn't the right one