If you are going for realism here, and want more than a hand-waving, techno-babble explanation, then you will have to do some serious thinking to create a world where sirens are possible.
First of all, it unlikely that a bird would be able to evolve visual mimicry of a mammal. Convincing mimicry would have to be very good to be useful at all. The bird would have to pass through a very deep low-fitness valley before the mimicry became useful.
A song that attracts only men is emphatically not possible. There may be a slight statistical difference between men and women in the tones they can hear. But this is an average difference. Some men (and most children) will be able to hear the high notes. Some women (especially older ones) will not. There is no way the siren will be able to specifically target the adults of one gender. At best, the siren song might attract a few percent more men than women. There will be lots of false positives.
Then there is the issue of mind-control through music. Music of course has some emotional power over a person, but there is no way in hell a song will make someone walk off a cliff or into the arms of a flesh-eating bird. The story of Odysseus tying himself to the mast of his ship is mythology. If you are going for realism, you just can't have this.
Also, cross-species reproduction does not work. There are no biological examples of this. Hybrids between distant species (say mammals and reptiles), is also totally out. If you are going for realism, you will have to be creative.
OK, sorry for raining on your parade. But I have good news. You can make this work and your world will be richer for it. You will need to think back a long ways, all the way back to the Carboniferous 400 million years ago and to the common ancestor of mammals and dinosaurs. This is where your story will need to start.
Plodding through the Carboniferous swamps was a species of ugly tetropods called pederpes. In the real world this species eventually diverged into the sauropsids (the common ancestors of dinosaurs and birds) and synapsids (the common ancestor of mammals). But in this alternative world, this is where the first sirens appeared. One population of pederpes began to mimic the female mating calls of another population, lure males into their groups and then killing and eating them. This prevented the other males from taking their females, it also provided food. Gradually the populations of siren and non-sirens pederpes diverged until they could no longer interbreed (became distinct species). But they did not evolve independently afterwards. Instead the sirens evolved under the constraint that they must be able to mimic the appearance and song of their prey. The sirens continued to evolve and speciate in parallel to their prey retaining only a superficial similarity. Over the next 200 million years the sirens evolved into dinosaurs and birds and the non-sirens into mammals. Most of the siren's descendants lost the siren trait and began to evolve freely. But some retained the siren trait and continued to optimize their mimicry.
The sirens could not interbreed with the non-sirens. But they could mimic their mating calls, their pheromones, and their appearance. The sexual dimorphism of the sirens decreased to almost nothing, such that both siren males and females resemble the females of their prey. The sirens are still sexual. Becoming asexual would vastly decrease their evolutionary potential and would not be viable for the sirens, since they must constantly adapt to mimic their prey.
The sirens seduce the males, and have sex with them, lulling them into complacency. Just after the males climax is when they are at their weakest, and is when the sirens kill the male.
But killing the males that are vulnerable to your song is a bad evolutionary strategy. It will result in the males that are not vulnerable reproducing more, leading to the prey evolving resistance. The best strategy would be to domesticate the non-sirens. To use the semen of the sensitive males to fertilize the non-siren females. In this way, you can eat your male and his children too without them evolving resistance. Alternatively, the sirens might only target men who are beyond their reproductive prime.
Regardless of what you decide the sirens do with the semen, here is how your world looks. There are male and female sirens. Humans in your world mistakenly believe they are all women. Further, the sirens are not true hybrids or chimeras, they are as distant from humans as other birds. That they actually are fertilized when they have sex with a man is another misunderstanding. They either mix the semen into the gravy they serve the man with, or use it to fertilize their stock of human women. Also, there are several other species of sirens that mimic other animal species.
Sirens are rare in this world. Humans are smart and will rapidly adapt to the wiles of the siren if they become too familiar with them. The song of the siren is not magic. It is beautiful, and can easily attract the lonely traveler, but is not irresistible.
That the song is magic is a misconception. That the song only attracts men is a popular, but completely false, belief. The truth is, the sirens just don't sing for anyone and will only try to attract a man who they think is vulnerable (e.g. a lone traveler or small group). Since the sirens rely on sexual allure that is adapted to men, they don't often sing for women. But a woman would find their song just as beautiful.
Also the sirens don't have to eat human meat. They can eat any meat. They may be omnivorous as well (your choice). Depending on how intelligent they are (and you have a lot of leeway here also) some sirens may choose not to eat human meat at all. Generally eating members of highly-social apex predator species that glorifies war and vengeance is a bad survival strategy. The sirens would likely be able to integrate very well into the human world if they chose. Some might adapt their talents in brothels or become bards.
