Sexual innuendo is a hard topic to stay on top of. As a humor tool, it stands erect in the English language. While there are no hard and fast rules as to what constitutes sexual innuendo, many people have mass-debated over the ins-and-outs of the topic, and now the general principles at the root of the topic are firm and well-rounded. However, full penetration of the subject requires that the reader take a long, hard look at the target and be a cunning linguist in order to avoid limp phrases and imbibe the phrase with a large handful of meanings. The topic can become hot by attempting to grasp it, and the more one experiments with it, the more interested they become. Also, as the language changes innuendos must change in order to fill the newly created holes and satisfy listeners.
Mary: I love this big stick
John: But mines bigger and harder
Mary: It wasn't meant as sexual inuendo