Banana trees have male and female flowers, on the same plant.
Many vegetables and fruits plants are polyploid: this is a "genetic defect" which produces additional copies of chromosomes. More copies means more proteins which means larger plant/larger fruits. But then the plant is also more dysfunctional (so it needs help [active cultivation] and often sterile), and for some fruits, there is also a selection to sterile varieties: we can eat all the fruit (but unfortunately most of species will give a smaller fruits if the flower is not properly fecundated).
The genetic of bananas is interesting, and it seems to prove that we (human) started to cultivate vegetables a long before the "Fertile Crescent" cultivations. Also the etymology of banana names is also interesting, because it seems to follow the genetic diffusion of bananas, according the original species.
Read also the answer of winwaed, which give more details of the current varieties of bananas.
As winwaed wrote, on some places there are wild bananas, and other species and varieties are cultivated. Many of them have seeds. Fruits born on female flowers, but not having seeds, doesn't mean that the plant has no male flowers (or part of flower as more common), not that there are no male plants around.