Sex is everywhere in Tristram Shandy, but it's constantly being pushed aside, interrupted, and deferred. You could even say that Tristram Shandy doesn't contain one completed sex act—like the novel itself, sex never quite reaches its goal. In fact, the novel is one big literary coitus interruptus—or maybe all the representations of coitus interruptus are metaphors for Tristram's inability to finish the novel. Whatever the answer, writing and sex clearly have a lot to do with each other. They're both kinds of creation: the result of sex is a child, and the result of writing is a novel. Both offer a kind of immortality and, in Tristram Shandy, the immortality that's produced is wounded and incomplete.
In Tristram Shandy, friendship is a better way of being close to someone than sex is.
Tristram's inadequacy at sex, or his perceived inadequacy, is responsible for his digressive writing style.