I don't agree with that definition

It's oversimplifying, inserts a false dichotomy of 'not sexual' and 'sexual' as some sort of 0/100 absolute, which is ridiculous because they more accurately describe directions of predominance.

Don't ask me to prove a quality you have necessitated on your own.

That said, in regards to 'not sexual' one can assume by default that something is 'not sexual' until there is a good reason to think it is. For example: I assume rocks are not sexual. I don't have the capability to prove a negative but that is a fair assumption because rocks have never demonstrated sexuality.

Of course, problematically again for your request: sexuality isn't just a thing present to living forms. For example, Michelangelo's David is a rock, but to me it is sexual because he looks so muscular and sexually attractive.

He is actually not in a provocative pose, and he is just nude. However I view him as sexual if I am in the mood to notice his nudity. This is based on context, and in our society we tend to sexualize many things, so there is often a sexual context associated with nudity.

To make a comparison to this: if I posted a picture of David on a gay porn forum, it would be sexualized.

Pictures are not sexual because they are images, data. They can depict things which are sexual or be interpreted sexually though, that would be true.

So, even though I never implied something has to be asexual to be sexualized (I don't agree, you can sexualize something already sexual to make it more sexual, or desexualize something without it resulting in something not sexual) I still give a good example here of how you can make a good case for it.

Don't ask me to prove a negative, I don't need to. Some things are common sense, like the asexuality of light.