What does "If she smokes, she pokes" exactly mean?

The "she pokes" part means "she engages in sexual intercourse of a heterosexual variety". There is an implication of doing so more readily or enthusiastically than may be otherwise expected.

You may wish to consult Urban Dictionary on the topic. But then again you may not.

Women smokers were considered to be more promiscuous - mass culture with the advent of the roaring Twenties, but traveling across the Americas prior to that through literature post Civil War (see Elmer Gantry) - which was really an American version of an idea that had been circulating via French Postcards visually.

She smokes, she will have sex

Actually, the phrase should be, "if she smokes, she ALLOWs poking."

If she is "loose" with regard to smoking, she is likely to be "loose" regarding other matters such as sex.

But the original phrase is an abbreviation.