Either man sins by free choice or man is forced to sin. If God forces men to sin then God is responsible for their sin. If God does not force them to sin then they must sin by their own choice. Any other conclusion is absurd and ridiculous. Seeing that man is capable of choosing that which is wrong, man must, to protect the rights and welfare of all, be put under moral obligation to choose that which is right.
Moral obligation applies directly to our ultimate motive, intent, or purpose.
Moral law is a rule for moral action with consequences. It is a law that moral agents must voluntarily keep. They thus prove that they know that there is a standard, for they are, by saying such things, appealing to that standard. The moral law is non other than the love law. Love has nothing to do with feelings, it is willing the greatest good of another with no selfish motive.