When you think about it, it's a weird thing to call a someone you are having sex with. Or would like to be having sex with. Is it a fairly modern usage?
I would love to be shown early examples in poems, songs, etc. I'm mostly referring to the English language, but other languages would be interesting as well.
There's a couple mentions of it in older love letters, one in the late 17th century; in Aphra Behn's 1694 novel, Love Letters between a Nobleman and his Sister, Philander cannot bear the thought that something might afflict his baby.
"BELIEVE me, charming Sylvia, I live not those hours I am absent from thee, thou art my life, my soul, and my eternal felicity; while you believe this truth, my Sylvia, you will not entertain a thousand fears, if I but stay a moment beyond my appointed hour; especially when Philander, who is not able to support the thought that any thing should afflict his lovely baby, takes care from hour to hour to satisfy her tender doubting heart. My dearest, I am gone into the city to my advocate's, my trial with Monsieur the Count, your father, coming on to-morrow, and it will be at least two tedious hours ere I can bring my adorable her."^1
As to why this term is used, it seems to be down to the fact that men value neotenous traits in women, such as wide eyes, smooth skin and high voices. Why do men value these traits? One obvious answer is that these characteristics normally identify with youth, an obvious plus in terms of fertility (a younger woman will be fertile for longer).
I appreciate this answer was fairly brief, and I will look for some more examples when I get home. Feel free to ask any questions.
Not quite the same, but puella is used as a term of endearment in Latin. Mea puella can be used more or less as "my girlfriend" is in English -- it's a common enough usage, for example, in the love poets of the first century BC. This is at least somewhat to the point, as the Latin term puella really means girl, or little girl -- it's a feminized diminutive of puer, which means boy. I do need to point out that it does have various other uses -- it could be used for a slave of whatever age, for example.