When a man "blows his wad," at least two different things could have occurred: he either spent or lost all his money in a wager (his wad), or he spent or ejaculated his sperm (his wad). Thorne goes on to suggest that since at least the 1950s "blow," in this context, is a euphemism for "ejaculate."
2001, Peter F. Murphy, Studs, Tools, and the Family Jewels: Metaphors Men Live By, Madison, Wis.: University of Wisconsin Press, page 44