Human women are smaller than human men. Most women see this as a turn on, but still, wouldn’t it be nice to see what it’s like on the other side? With a Gardevoir, the human woman can experience this, as Gardevoir are smaller than even the daintiest of human women.
Then there’s the Gardevoir perspective. Gardevoir would like human men, but they’re nothing a Gardevoir wouldn’t have seen before. An average man would look like a bulkier Gardevoir. A well built man would at best look like a smaller Machamp. Human women, on the other hand, would look incomparable.
The most appealing things about Gardevoir are their figure. No human can look as petite as them before becoming horribly skeletally thin. They have the bodies of angels. I think Gardevoir would feel the same way about human women. No Gardevoir could achieve the level of curviness and fullness of even an average human woman without becoming obese in the process, they simply don’t have the body type for it. To a Gardevoir, a human woman would have the body of a goddess.
They were made for each other.