Someone please explain to me the logic of this pairing.

DxG as a couple has more logic than this question!

Dungwen is not illogical by any means and was not created to appease fans. I have no idea where you are getting your facts, but they do not make sense.

Sidenote: DxG are still together (and we're going into season 5) and it has been more than 2 months, so they beat your odds. Just sayin'.

Duncan and Gwen have a lot of chemistry. They started out as friends first....slowly getting to know each other and everything about one another, which is more than Duncan and Courtney ever did. They have similar tastes in movies, likes, and interests. They actually have different fashion sense: Duncan is a punk, whereas Gwen is a goth.
Gwen DOES NOT change Duncan. Gwen DOES NOT control Duncan. Gwen IS laid back and NOT uptight in Duncan's business 24/7. Duncan can be himself around her and doesn't have to be nagged/bitched at for not doing something up to Courtney's par. Duncan and Gwen know more about each other, actually care about each other and their safety (rewatch season 3), and have a similar sense of humor.

I could go on, but I know I'm not going to change your mind. You like DxC and therefore will not like DxG.....ever. If you need a list of equivalents, similarities, and foils between the two, go to the Duncan and Gwen wiki and wiki blog section and others will share those points and reiterate what I've said. There's also some more good parts too......but it's a DxG fansite dedicated to how awesome the pairing is. It's a good read. ;)