The title tells us what the poem is about, and the three short lines then twist it into something stranger. The speaker says he is "at my peak," a phrase that sounds like triumph, yet the title suggests the opposite: a body that won't respond. The poem lives in that gap between how things should be and how they are.
The comparison to God is the surprising part. God is often imagined as all-powerful, but here he is "not present to my wife," a God who is distant and silent. The speaker seems to be saying that at the very moment he should be most fully there, he is absent, just as God can feel absent to believers. "Present" works two ways: physically there, and emotionally attentive.
The mood is dry, rueful and a little sad. The speaker appears ashamed, but the odd, almost joking comparison keeps the poem from self-pity. It admits failure with wit rather than despair.