In just four lines, the speaker describes his wife in terms borrowed from church, and in doing so blurs the line between physical love and religious faith. She is "an oiled leg of church," a strange, striking image that suggests both a pillar holding up a building (the title points that way) and a body glistening with oil, perhaps echoing anointing as well as touch. She is also "a long psalm," something to be recited slowly and returned to.
The last two lines carry the central idea. Sex is "the red letter" he preserves among his prayers. Red letters in some Bibles mark the words of Jesus, so the phrase suggests something especially valued, set apart from the rest. The speaker doesn't treat desire as separate from devotion; it sits right alongside it.
The mood is hushed, reverent, and sensual all at once. He sounds grateful and committed, treating his marriage as something sacred and worth guarding.