Catching a Gardevoir is a bit more complex.
The Ralts-Line family are psychics and are only attracted to people with pure intents.
A Ralts is weak, so it's not as difficult to catch and raise one. It's pretty much like picking up a human baby left on the street. It will eventually develop a connection with you and you will have a new partner.
Kirlia, Gardevoir and Gallade, however, are a bit more complex. Since they are older and have a better control over their powers, as well as better understandings of huans and their emotions, they will not hesitate to struggle against their capture, maybe even resorting to using their powers as means to self-defense.
Should they be caught, however, that's it. They're considered the Trainer's property.
As close to humans as they are, they are still treated as Pokémon and will continue to be considered as such, sadly.
This is not the case with only the Ralts-Line thread, however. Most human-like Pokémon, as well as all Psychic types posses either equal to, or greater, human intelligence, which should put them on the same level as humans.
However, the law still considers them Pokémon, which are albeit supposed to be equal to humans, are still being controlled by the humans.
To put it simply - yes, it would be immoral to catch a Gardevoir against her will since she possesses human-like intelligence that is either equal or greater than humans', but it's not illegal as she is still considered Pokémon and the laws allow you to catch and train her.