It is not a “false distinction.”
Life begins at conception.
Not before.
The Onan incident clearly refers to methods that waste the seed, such as barrier methods.
It is wise and prudent to oppose abortifacient hormonal contraceptives. But its not Christian to make this false distinction and accept barrier methods.
Hormonal contraceptives are immoral because by nature they are contraceptive. They are doubly immoral because they are also abortifacient.
Your dismissal of Calvin's wisdom on this issue is part and parcel of mainstream Protestantism's falling away on moral theology. Thankfully the best and brightest in the OPC are swimming against this tide.