Oh, and by the way, Mr. Science, the detailed reason scientists have this concern is because an insufficient population in bisexuals means the loss of all the possible instantaneous variations in genetic composition that recessives and other exchange permutations with similar effect to the recessive mechanism make possible. In other words, too much the capacity of a species to cope with it's environmental changes by altering it's physical capacities through genetic reformation is lost.
In effect, a bisexual species is really, in comparison to unisexuals, a vast conglomeration of species always ready to zip out from the drawing board in response to the environment. Externally caused mutation isn't really the biggest player in the DNA story, if it ever was.
Lets look at some more of the details of this argument: The existence of recessives&such is quite bit more dramatic a bit of evidence than most people at first realize. It is not just that a line of linear descent can have variations with each 4th offspring. Because large populations mix, the net heritage of a bisexual population doesn't have to be reflected in every member. There can be recessives&such that are totally unavailable in any particular mating, that could still eventually be recaptured by one's distant offspring, through judicious subsequent matings.
To appreciate more fully what is going on here, you have to divorce yourself from the linear tree of descent creationists are so fond of arguing about. A modern bisexual species is a gene corporation, no one member of which is a fully empowered representative of, with a broader survival mandate than any one member represents.
The existence of dog breeds is a sort of butterfly-pinned-to-the glass picture of this fluidity in bisexuals, made obvious by the application for a few thousand years by humans of various diverse pressures nature only generally applies one at a time.