I feel like this question deserves some sort of SJW gif because of the gender stereotypes involved in the question so here is my queen: http://i.imgur.com/zN3SVVC.gif
If you look at the list of Pokemon that can only appear as one gender, you notice that many Pokemon are exclusively one sex in order to juxtapose another Pokemon, which is usually the opposite sex (e.g. Volbeat and Illumise, Glalie and Froslass, Braviary and Mandibuzz, Wormadam and Mothim...).
Gallade and Gardevoir were added to that list in Gen IV. Game Freak would have realised how traditionally feminine Kirlia and Gardevoir were, so they made a more masculine counterpart to contrast. With the skirt and the hair? and all, it's safe to assume Kirlia and Gardevoir have a pretty feminine presentation. However, it's safe to assume that Ralts has a presentation more like a helpless infant. Now, Game Freak can't make Ralts have a 50/50 gender ratio and Kirlia have a 100% female ratio because that'll result all male Ralts changing gender upon evolution, which would be odd. As Ralts is much more common than Kirlia, it would be more sensible to give Ralts a neutral gender ratio to match its neutral appearance. This is unlike Smoochum and Jynx, who both look pretty traditionally feminine (although Smoochum matches the infant look as well, I guess it looks more feminine than neutral. Jynx technically is encountered more often though.)
I also completely forgot to talk about that Gardevoir is actually based on a Japanese knight. Gardevoir's Japanese name is romanised as 'Sirknight', so if anything, Gardevoir might present in a more masculine manner in Asian cultures. A 50/50 gender ratio keeps everyone happy here! Thanks Astronautical for pointing that out. ^-^;
This all being said, gender ratios are highly illogical in Pokemon. Game Freak probably don't think deep enough as to see the consequences of the gender stereotypes they are upon their Pokemon (and hence their audience)... they really don't care that much. Mr. Mime has a 50/50 gender ratio and contrary to my point before, Azurill has a 25% of changing gender upon evolution for some reason. They can fix the Azurill issue, but if the Mr. Mime issue were to be fixed in later generations, all female Mr. Mimes would probably be restricted from migration. Are these ratios meant to be trans* acceptance statements? Unlikely. But they can be if you want them to be.
My point is, don't overthink the gender ratios in Pokemon, especially the 50/50 ones. Let Pokemon identify as they want. Let them express who they are. ~
TL;DR: Ralts has a more neutral appearance and is more common, thus in order to avoid more Azurill gender problems, Kirlia just follows the 50/50 ratio of Ralts. Plus Gardevoir is based on a Japanese knight which is a traditionally a masculine gender role.
Hope I helped. :)
Source: The gif