Robinson Crusoe is a very specific story, having to do with a man who is shipwrecked on an island for 30+ years. Although Defoe does mention Crusoe's parents (including his mother) at the beginning of the tale, as well as a woman at the end who had the handling of his estate should he not return, there simply are not a great many women in this story, and I can't think of any good reasons why there should be. Loosely based on the account of a Scottish sailor who was stranded on an island off the coast of South America, Defoe wrote his story as an adventure as well as of a selfish man coming to terms with what was truly important in his life - the love and mercy of God.
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If you are curious about stories in which there are men and women on islands, I recommend that you take a look at a very good novel by Joseph Conrad titled Victory. There is also Pitcairn's Island by Nordhoff and Hall, a part of their Bounty Trilogy. If a man were marooned on an island with an attractive woman, it wouldn't be an adventure story but more of a fantasy. He might never want to leave the island. Then, of course, they would probably be producing babies, and you would have a Swiss Family Robinson type of story. If a man were marooned on an island in the South Pacific and managed to acquire a female companion, she would probably be of a different race, and this would perhaps lead to a plot dominated by the theme of interracial marriage.
It seems to me that Robinson Crusoe is mainly a survival story. He really lucks out by having access to a shipload of supplies before his wrecked ship finally sinks. He has all sorts of tools, plus muskets and pistols and plenty of ammunition. He also has some animals which he can breed for an unlimited amount of meat, milk, cheese, eggs, etc. And furthermore the island is a tropical paradise with lots of fruits and game. So he can not only survive but survive comfortably. Providing him with a girlfriend would be stretching credibility too far, and I think Defoe knew it. We enjoy reading Robinson Crusoe because he has a pretty good life as king of his island--but he has to have some hardships, some problems, or it would be too much like a pure fantasy.
I think its relevant with the eıghteenth century. Middle class women have to read, entertain themselves and marry, multiply. ıt would be very absurd in thay era to see a woman on an island. and advanture is for males and women' adventures are reading of men made fiction. so Defoe, as selfish as as 18th century gentleman. Robinson repeats himself as a sinner and finds himself in a deserted island. adding a female figure makes him kinda Adam and the story of Genesis comes to mind. he's the king of his world. and there is no need to women. maybe its kinda escaping from the ordinary world for salvation from temptation. even in a patriarchial society, one of the most noticable woman character-captain's widow- helps him by preserving his money.this shows the feminine loyalty again.
these are my inferences. but are there any other points that you can see?