Several reasons... Pimps who prey upon young women, slavery, dirty old creeps who can't think with the appendage at the upper end of the spinal column and spread diseases.
There are more, but I don't think you are ready to have that discussion with me.
One would get the impression from your post that sex were designed for marriage and family.
How dare you declare such a thing, in the midst of our self-loving, self-loathing hedonist culture! Don’t you know we are free to destroy our civilization — “just because we can?”
/sarcasm