Mermen are not necessarily hideous, but they don't fit the stereotype that we've developed of them. Instead of having a "human" upper body and an aquatic lower body, these fine specimens are fitted with the head and torso of a whale. The arms remain whale-like but, have grown into hands and arms. The legs are just human legs.
Scientifically, the leg adaptation doesn't make sense. Why have legs when you can have fins? Good Question. Before humans polluted the oceans, mermen had one leg. The feet of the legs were pointed out, much like any fin you would see on a fish today.
A very potent string of acetaldehyde was dumped into oceans all around the globe. It had originally been thought to cure cancer. Once hundreds of animal trials and a handful of human trials gruesomely failed, the project was abandoned. The huge pH change and chemical breakdown in the oceans caused these mermen to mutate. With less animals preying on them, even the weaker mutated ones (the ones with two legs) survived. For some reason, these two-legged mermen were thought to be stronger than their kin and therefore, got better mates. This shift caused the all the one-legged mermen to die out, leading to an entire species of just two-legged mermen.