For one thing, definitions and labels, at least in sexuality, are personal. Some people use them to make it easier for others to understand, others don't. I'm kind of in the middle, "homosexual" is pretty much what I am, but I've slept with women (lost my virginity to one), and I don't just like a good looking guy. But I still consider myself gay, and would rather people treat me as a gay guy. For example, not go "When are you going to get married to a nice girl and raise a kid?"
In that vein, someone can call themselves asexual as much because that's how they feel as I do for homosexual. Maybe they have sex with someone once in a while, but they don't "enjoy" it for anything but the literal stimulation, a la the masturbation angle, they'd be just as happy with a robot as with another person and the circumstances came up this way. Maybe they DO enjoy it for the other person, but on a purely "Hey, this guy's a nice dude, I wanna make him feel good," rather than "Man he's hot, let's get down to business." So an altrustic angle on sex rather than the normal motivations.
It doesn't -really- matter what their actions are. What matters is what they prefer to label themselves. If they call themselves asexual, treat them that way. Don't ask them when the last time they got laid, don't try and hook them up with someone, don't go "Hey, he's hot, right?" Like I said, maybe they have had sex or maybe it's an occasional thing, but nearly everyone does things occasionally that go against one of their chosen labels. That one moment doesn't nullify every OTHER moment where they go WITH one of their chosen labels.