You can’t know till you’ve tried it

Two points.
One:
Asexuality doesn’t mean not liking sex. There are asexuals who like sex, there are asexuals who feel indifferent about having sex, there are asexuals who don’t like sex and there are asexuals who have never had sex. And they’re all valid. Because sexual orientations aren’t about whether you have sex or not and they’re not or about whether you like having sex or not. They are about who you are and aren’t sexually attracted to.
Asexuality is the sexual orientation that excludes all genders from that “attracted to” list. You can know who you are and who you aren’t sexually attracted to without having sex with them.
Which leads to my second point.
Two:
If you use that argument, that an asexual person can’t know they are not sexually attracted to people without having had sex with them, it implies that you can’t know it yourself. You’d have to “try it” with members of all genders to exclude them or include them. That doesn’t make much sense now, does it? Again, you can know who you are and who you aren’t sexually attracted to without having sex with them.

See also You can’t be asexual; you’ve had sex