They say the Bible claims that anal sex is a sin.

Actually this popular myth is a misconception. Many Christians frown upon anal sex as in their opinion the Bible rejects it. In fact, the Holy Book denies anal sex but that one that occurs between two men.

Briefly saying the Bible condemns homosexuality. There is no statement in which anal sex between a man and a woman is forbidden.
Just have a look at these passages:

“The virgins of Jerusalem have bowed their heads to the ground,”.
This is the position a woman should take to receive anal sex.

In the Song of Songs, the man urges his woman to allow him to enter her from behind:
“Draw me after you, let us make haste.” (Song of Solomon, 1:4)

They say the Bible sees anal sex as dirty.
Let's remember the quote: “To the pure, all things are pure.” (Titus 1:15) God has created a body and all parts of it are perfect and clean. The Lord has also created a human body for feeling pleasure. Anal sex is a way to feel this enormous satisfaction and experience great sensations.

And even though the anus is mostly used for elimination of internal wastes, it's not unclean as one believes. Elimination is a natural process of an individual's body and eventually anal sex can not be dirty.