The mermaid sounds truly outraged and her words make you stop. True, she could be lying. Furthermore, you are not at all a whiz in mythology but you know that gods don't really care about their kids. Rarely, even. So even if her father is Poseidon in person, there are no chances he would come to the rescue. Besides that mermaid seems to have a lot of sisters. Would the god really see the difference and bother himself with getting one back when he has got so many other children? No, probably not indeed. However... However, there is still a slight chance that Poseidon might come forward and that... That would be bad. Very bad. Angry gods are hardly easy to reason or even deal with, after all. And gods like transformations, notably humiliating ones for those that anger them. So the arrival of the sea god would be very scary and could have a lot of disastrous consequences. So instead, you decide to play upon an idea you've just had.
"All right, all right," you grumble. "But you are very beautiful and I really do love you. So how about we make a deal?"
You stop and set the mermaid back upon the sand.
"A deal?" she sputters. "You've got some nerve..."
"Transform me into your male ideal and I will release you for good. Because I know you'll find me super attractive and wish to return to me. Is that possible?"
She smiles.
"Fine. A deal done is a bargain made."
Her eyes then flash with sparkles and suddenly you feel your body transforming.