You can think of asexual as a word with a hole in it. Sexual names one whole side of life — bodies pairing up, attraction, making young from two parents — and the small piece a- in front says: that side is absent here. What is left over depends on what you are pointing at. Point at a cell, and what is absent is the second parent, so the cell splits and copies itself. Point at a friendship or a picture, and what is absent is desire, so only closeness or plainness is left. Point at a person, and what is absent is the pull of attraction, while the caring and the love stay exactly where they were.
From the word’s history
The word is built from a- (lacking, without), which came into English from Ancient Greek, plus sexual, which goes back through Latin sexualis to sexus. So the shape of the word is exactly the shape of the meaning: sexual with the a- of absence in front of it.
這個字由 a-（沒有、缺少，經由古希臘文進入英文）加上 sexual（性的，經拉丁文 sexualis 回到 sexus）組成。字的結構就等於意思的結構：在 sexual 前面放上一個表示「缺少」的 a-。
Wiktionary etymology tree supplied in this packet (source_ref kaikki:3f6f69a190036ae4:629462817): Ancient Greek ἀ- > English a-; Latin sexus > sexualis > English sexual > asexual.
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