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One difference between us and the animals is that we are made in God’s image, and we are also designed to be a temple for the Holy Spirit. When we commit adultery, we sin against this temple in particular. I Cor 6 “the sexually immoral person sins against his own body. Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God?”
Sexual sin destroys attentiveness to God’s commands and ultimately prevents salvation. Animals’ sexual behavior cannot change their attentiveness to God. Eph 4 "They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart. They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity. "
Sexual sin dishonors and destroys marriage which is a special, honorable creation of God’s for our spiritual benefit (that our prayers be heard, to make us fit to serve the church, to bring about salvation of our spouse.) Marriage is only for humans. I Pet 3 “husbands, live with your wives in an understanding way…so that your prayers may not be hindered.” Heb 13 “Let marriage be held in honor among all, and let the marriage bed be undefiled.” I Pet 3 “even if some do not obey the word, they may be won without a word by the conduct of their wives, 2 when they see your respectful and pure conduct.” I Tim 3 “Therefore an overseer must be above reproach, the husband of one wife…for if someone does not know how to manage his own household, how will he care for God’s church?”
Adultery requires encouraging desire in a co-participant. Convincing another to sin, especially to their spiritual death is a special evil. Animals’ mating rituals are not decisions like ours and cannot lead to a wrong partner. Prov 5 “The iniquities of the wicked ensnare him, and he is held fast in the cords of his sin. He dies for lack of discipline, and because of his great folly he is led astray.” I Cor 8 “By sinning against your brothers and wounding their conscience when it is weak, you sin against Christ.” Matthew 18 “whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened around his neck and to be drowned in the depth of the sea.”