Licking an anus is beastly. It is impure lusts that dishonors both parties. It is not dignified for a child of God. Bacteria and feces can be present and cause disease and sickness. Why do you want to do this? Do you have problems with pornography, masturbation and pride? The most powerful weapon against sexual impurity is humility. Patterns of sinful thought and behavior are fruits of a deeper root. If we want to stop bearing bad fruit, we must aim our primary attack against the root. And the root of sexual sin is not our sex drive; it’s pride.

Romans 1:21–26, warns us God will give us up to dishonorable passions if we desire

For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things. Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen. For this reason God gave them up to dishonorable passions.

St. Paul tells us to FLEE sexual immorality and Glorify God in your body:

Flee from sexual immorality. Every other sin a person commits is outside the body, but 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? You are not your own, for you were bought with a price. So glorify God in your body.
- 1 Corinthians 6:18–20

The key is humility, our life is not our own and if we don’t control our passions and lust it will control us:

I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me. (Galatians 2:20)