Some forms of contraception do in fact act as almost "home abortion kits" where the very beginning stages of the embryo will be flushed out.I apologize for arguing earlier. But I thought I'd just give my opinion on the subject.
Preventing pregnancy is okay.
Destroying an existing pregnancy is not.
Now I don't really know enough about birth control to know if it works before or after conception, but I think condoms and such are not sin.
But the reason why contraception is explicitly a sin is obvious; it dismisses all possibility of using God's gift of sex the way he authorized it. It's disobedience.
When contraception became more popular and accepted by people, God didn't decide to change His mind. The only ones who did were the many of the Christians. Most of them out of convenience. But the destruction birth control has had to society is far more great than people realize; an increase of divorces and infidelities, an increase of objectifying women, and a far bigger increase of abortion.
Yet despite all these things, people refuse to admit that the condoning of contraception is strongly part of it.