Um, you have to marry the girls off before menses start in order to make sure of the paternity of any kids they have/to make sure that if the girl does become pregnant in any way, she has a husband to take care of her/the kids.
In a world without birth control, it protects both parties' interests.
As for slavery, slave holders genuinely believed (for the most part) that slavery was a moral good.
On Being Brought from Africa to America
'Twas mercy brought me from my Pagan land,
Taught my benighted soul to understand
That there's a God, that there's a Saviour too:
Once I redemption neither sought nor knew.
Some view our sable race with scornful eye,
"Their colour is a diabolic die."
Remember, Christians, Negro's, black as Cain,
May be refin'd, and join th' angelic train.
--Phillis Wheatley
Folks in the 1800s weren't going around saying, "Oh my God, we are
monsters, slavery is an abomination." No, they actually believed that slavery was a
good thing, because they rescued a bunch of half-naked savage pagans from destitution, and gave them clothes, food, civilization, and Christianity. They were saving lives and saving souls from the fiery furnaces of Hell. Abolitionists in the early 1800s were seen as nuts, and the early slave rebellions were led by folks who heard voices in their heads or were otherwise disconnected from reality. Whites regarded these rebels not as fighting for their freedom, but as homicidal maniacs--and other slaves helped end these rebellions.
It wasn't until the industrial revolution started making slavery less economically valuable that the idea that slavery is bad began to spread.
I just wrote a little essay on this:
http://users.livejournal.com/little_e_/360616.html