How? Asexual has never meant "I do not feel sexual pleasure". If I'm not in the mood for sex and my gf comes over and starts jerking me off that doesn't mean it's going to stop it from feeling good. In the same vain; if somebody is never in the mood or attracted to someone, they can still get off all the same.
asexual until recently was always "I don't feel attraction to people". How do you like sex but not feel attraction to the person having sex with you? If your truly not in the mood, someone touching you wouldn't provoke a response. If someone I wholly felt was unattractive try to interact with me in a sexual manner, I would feel disgust. It doesn't make sense for there to be a "spectrum" for asexuality, you either are attracted to people or not. Anything outside of that would just be one the hoe spectrum.
Because sexual stimulation can feel good even if you're not horny?
Not true at all. I get hard all the time without being horny, as is normal. And if some hypothetical fat greasy woman came up to me during that time I'd still be able to have sex with her even if I wasn't into it. Maybe that's dependent on the capability of someone's crotch but people having sex/masturbating when not in the mood is one of the most common things in a relationship.
A lot of asexuals ARE sex-repulsed. Those ones usually only do it to please their partner so they'll have a romantic person in their life that won't leave them. Some asexuals just don't find sex enjoyable because they find it boring. Some just don't like physical contact in general. It's not some black & white binary title.