Why not mShep if he's with Kaidan, though? That one takes him like 3 years and some serious emotional connection before he shows attraction.
Well, one reason is Shepard's reactions go against this:
6. Flirting doesn’t make sense to you.
Maybe it goes right over your head, or it makes you uncomfortable. The idea of flirting seems pointless—why do that when you can have a proper, deep conversation and get to the know the person instead? Maybe when someone gets flirty, you just have no idea how to respond and would prefer to run in the other direction. You feel frustrated that you can’t just talk to someone normally—why do they have to hit on you? Your friends point out that people were flirting with you or checking out out later, or you realize only belatedly. You tend to avoid situations, like parties or clubs, where you can expect lots of flirting, and if you do, you’re either on high alert or happily oblivious.
Shepard doesn't experience any obliviousness or discomfort from Kaidan's flirts and does some flirting himself, thus going against an aspect of demisexuality.