"So explain again how this justifies cherry-picking the laws in Leviticus and Deuteronomy?
Why are sins such as murder, theft, adultery and homosexuality so much more egregious in the eyes of certain Christians than, say, working on the sabbath, back-talking to your parents or eating unclean foods?"
First of all speaking as a Lutheran as opposed to a Catholic, all sins are equal in the eyes of God. It is another human sin to catergorize certain sins as worse than others. Secondly the "cherry picking" mostly involves the fact that many Old Testament Laws were more clearly explained by Christ (such as the Law about working on the Sabbath).