Two problems with this, one, the characters do get married often (for instance Godai Yusaku and Otanashi Kyoko being good examples)
And second, most of the manga are about kids 16-19. Most of them are too young to marry. Best term for most of these girls is "jailbait"
And what rational guy whould want to marry Akane Tendo? Even if she were legal. Her main rival, Shampoo, is even weirder.
And if the girls are strange, don't forget the guys either. Sesuke Sagura for starters.
And just image the offspring of some of the pairs. For example, could the the world survive the progeny of Sesuke Sagar and Chidori Kaname?