The New Covenant does not require us to live by the ceremonial law, concerned with how you should worship God, which is a large part of Leviticus. However the moral law there is an indication of what God requires of his people in a wider context - what true love means in practice. For example the list in Leviticus is contains rules for avoiding incest. The question is whether the list remain authoritative. For me it is not an absolute indication of what is right and wrong, but the presence on the list of things that all would agree are wrong means that it is not fair to dismiss the list as being things that are now never wrong. This is how Blackstone's argument comes across. My own rejection of the legitimacy of gay relationships is based on a far broader basis than any one verse.