Slang for a male who is the last heterosexual significant other to a female who proceeds to have exclusively homosexual relationships.
Originates from the myth/children's story about a little boy in the Netherlands who discovers a hole in a dike and plugs it with his finger until it can be repaired the next day.
The connection being the pun that "Little Dutch Boys" have a way of correcting the (erroneous) behavior of dykes.
Many Little Dutch Boys feel that they have caused their ex-girlfriend to "turn to" homosexuality, for whatever reason (not being great in bed is a frequent suspicion). Queer theory would claim that the female was a lesbian all along and only faced that side of her life coincidentally at a time during or shortly after the relationship with the Little Dutch Boy -- unrelated events which usually are misinterpreted by the ex as being connected.