My point was that if one accepts that consent and privacy are the two criteria by which one establishes whether or not sexual behavior should be legal, then there is no reason to prohibit a sadist from murdering a masochist for sexual pleasure.
How does inserting a penis into a rectum or mouth equate to murder? You are directly comparing murder to sodomy. These are your words, not mine. What consenting adults do (sexually), in the privacy of thier own homes is none of your business. Obviously, murder, cannabalism, forgery, drug sales, kidnapping, cheating on taxes, ecetera, are not sex.
If the criteria for sexual conduct are privacy and consent, they are not enough.
Why? Because there are any number of very questionable sexual practices would satisfy both rules: incest among parents and offspring, incest among sisters and brothers, among grandparents and grandchildren. Sexual sadists and masochists would satisfy both critera, as would those who would want to hold multiple wives.
If the criteria for sexual conduct is privacy and consent, all of the above could easily satisfy the criteria in question. My example of a sadist and a masochist engaging in murder for sexual pleasure is meant to bring the point home.