Other animals don't bother with foreplay of any kind. In and out, that's all there is for them. Man is the only creature that has made sex into more than procreation. Of course, some animals have a distinct feature that allows them to "clean" themselves, but can that really be considered oral sex? No, because they don't do JUST that part of them; they clean their whole bodies at once, that just happens to be a fringe benefit... Then again, there is the age old question: "Why do dogs lick their dicks?".. Because they can. Would man do the same, if our spines were more flexible? I'll leave that thought up to you.
Ignoring the mere aspect of having oral sex, let's think about why people want it in the first place? It might be because society doesn't look too favourably upon it. Most people like to be different, but I don't think that this is the main reason. Why does a man suck a woman's breast? The man really doesn't get any pleasure from it. But the fact that he is being intimate with a part of the anatomy that he doesn't have... it is intriguing. Same with the other areas. But then, why not have straight sex? Because everyone and everything has that. Man and beast.
As well, perhaps it is an expression of ultimate commitment to a person. To show them that you aren't afraid lick and swallow what they emit; that must say something about your feelings. Especially if you don't like the taste. You are making a sacrifice for that special someone. Boy, nothing says lovin' like a tongue in the ov-but I digress.
Then again, all people are different. What might seem sickening to some, might be a great turn on for others. Certainly sodomy is not restricted only to homosexuals, but not everyone would like to try it - even if it were with someone of the opposite sex. So for some, oral sex is something they enjoy. Getting or giving. (GETTING probably being more popular in standards of appreciation...)
Oral sex though, is something that has been done and thought about a lot. There have even been sayings made up about it.. (In regards to the size of tits) "Anything more than a mouthful is a waste" .. of course, anything less is a disappointment.
And also, there are certain problems that can come with too much oral sex. For example, have you ever tried to explain a pubic hair stuck in your teeth (To someone other than your partner.. or worse, to your partner who you haven't had oral sex with lately..) Or how about the horror of a girl who uses too much teeth.. Ow. Don't even think about that one. Ow.
But, whatever the case, oral sex is used by couples as a recreational sport. Do it when you're bored, without the hassle of birth control or running as great a risk of contracting VD. In a time when AIDS is foremost on people's minds, it's no wonder we're looking for sexual substitutes.