Protestants & Orthodox: How can us justify the use of contraceptives?

Yes, God gave us a brain, but He gave us a brain to choose what is right, namely to embrace His Will. Contraception slams the proverbial door in God's Face. His participation (with men and women) in bringing life into our world is disrupted when people rely on hormones and barriers to prevent conception. That's why NFP keeps God's participation within the relationship.