Oukoku e Tsuzuku Michi Himekishi ga Classmate! ~ Isekai Cheat de Dorei ka Harem~ Inside the Cave of Obscenity (MC is a slime though)
It might not be a novel but I suggest you to read the manhwa called "My Stepmom" it has romance, sex, a male lead and an interesting plot... I think. Edit: and yes, the stepmom plays a HUGE role in it
Can't really say I'm well read in ero novels but I remember Saimin Regulation being pretty good. Episodal chapters with a good sex scene each time (with the occasional non ero chapter to set the scene or a sex scene that will span multiple chapters). It's 1 guy with about 4 or 5 main girls with distinct personalities/body types/backgrounds/plot development etc.
LOL, but nope If anyone is looking for novels w/ such themes, they'll got better deals going to the sites that host such stories