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Arthur regretted his needs, regretted how he could not stay away from temptation. Lancelot certainly didn't seem to mind, but Arthur could feel himself breaking once the passion was over.
He knew it hurt Lancelot to watch him leave his bed almost immediately. He knew it would hurt Lancelot more if he knew that Arthur went to confess.
It was a twisted relationship, one that hurt Arthur even as it kept him sane.
It was after one night when he was kneeling at the altar, hands clasped together in an effort to pray, that he realized that this was the only time he attended church anymore. Arthur studied the crucifix before him. He knew the church, the religion in general really, bothered Lancelot. Lancelot refused to enter Arthur's chamber because of his crucifix. It bothered him that when all Arthur did was kill he would hang a carving of a dying man on his wall.
Lancelot put all his beliefs through the wringer, made him doubt himself and his life, and this was what brought him back to church? The need to repent, not for the killing but for the needs he couldn't ignore any longer.
Odd that Lancelot brought him closer to God.