I don't think Sten is asexual, really. He just comes from a culture that strictly believes in sex only for procreation. Qunari don't believe in romantic relationships and I get the impression they're not huge fans of casual sex, either. Getting in a relationship means the re-education camps for you. He's probably equally asexual to Sebastian. It's a religious thing.
There's evidence that one's libido level tends to be tied culture and life experience. A 30-year-old virgin from a culture that has little in the way of sexual expression could lack a sex drive.
Some priests report that after taking their vows and going several years without sexual or romantic interaction, they find they've lost interest in sex generally. And religious orders that have focused on abstinence (not only sexual) suggest that people experience intense cravings at first, but that over time people can find they no longer desire the things they once did.
Well, the people who live their lives only being attracted to drainpipes and bicycles probably get sick of YOUR whining, sooooo.....
That's really not cool. Being asexual isn't the same as wanting to have sex with a drainpipe at all.