I don't see the "romance" between Carl and Michonne as anything more than a mother/son one. Calling it a "romance" is just extreme.
Yeah, or we really need a "Just Kidding" emoticon so people like you don't take statements like that so freaking seriously.
But okay, let's take it your way. It's a matter of perspectives. From Michonne's perspective, of course it's a mother/son relationship. It helps her to have a kid to take care of that isn't a baby.
But let's look at the facts from Carl's persepective:
-No matter how motherly she is, Carl knows damn well she's not his mom and she's never going to truly replace his mom in his eyes.
-He's a preteen boy.
-The ZA has short-circuited any chance of him having a normal crush on a girl his age.
-Michonne's kinda hot, and she has swords.
Given all this, I have no trouble believing he has developed/is developing a crush on Michonne the more they spend time together and the relationship is therefore a "romance" - hence the quotes, although that should have been obvious - from Carl's perspective, given his limited experience.