Warning … The following material has adult content that may not be appropriate for small children, but most kids middle school and up already talk about this stuff all the time at school.
Simply, according to God, marriage and sex are related, connected, and exclusive. Sex as God intends it is for one man and one woman in marriage with the overarching purpose of oneness. Subsequently, by definition anything that contradicts God’s intent is sinful. Thus, sinful acts include homosexuality, bestiality, bisexuality, fornication, friends with benefits, adultery, swinging, prostitution, masturbating a person who is not your spouse, oral sex with anyone other than your spouse, anal sex with anyone other than your spouse, heavy petting outside of marriage, dry humping outside of marriage, rape, polygamy, sinful lust, pornography, phone sex with someone other than your spouse, sexual chatting online with someone other than your spouse, prostitution, pedophilia, incest, and anything else invented next to try and escape the clear teachings of Scripture.
Can you think of any sexual sins Mark has left out?