I think Ike Turner started the expression.
First, interestingly, McDonalds used "I'd hit it" in an advertising campaign in 2005, part of the "i'm loving it" campaign, leading to a blogger to post:
Mcdonalds wants you to **** it's sandwiches, in which he notes: Although I firmly believe that McDonalds is not advocating hot man on sandwich action, it is quite obvious that they did not do their homework on urban slang. If they had, it is likely that they would have realized "I’d Hit It" is not exactly the catch phrase to use when selling fast food.
OK, that was pretty funny, but not what I was looking for (although the site has a good McDonald's "I'd hit it" graphic someone could steal for the next Monica Belluci picture). Wikipedia added the following gem regarding the marketing attempt: The ad was quickly pulled from the internet after a flurry of controversy, which led to McDonald's publishing an apology in which they stated that the marketing department had misunderstood the term.
At least we can rest easy knowing they didn't MEAN we should be sexually aroused by a double cheeseburger.
(aside: There could be some confusion on this point, as the VeggieTales people at one point wrote a wonderful little song called "His Cheeseburger", where Mr. Luntz sings a love song centered around his desire for a cheeseburger, complete with what some might think is an overly erotic reference to him getting down on his hands and knees, where he'd "Clean the dirty cheese off just for you").
The Urban dictionary didn't give history, but gave the definition, for I'd Hit it they say Vernacular, commonly used by males, meaning, "I would like to have sexual relations with that female."
The thought of male freepers attempting to sound "black" makes my head hurt, so I'll just note that, if Bill Clinton was really the first black president, why isn't the phrase we'll all remember forever "I did not hit that woman"? Oh, now that I see it in print, I think I understand.
The phrase also has some more negative connotations (yes, it's possible), such as "I'll have sex with her but I'll be damned if I respect her." (I should think that a 2-second glance at a woman followed by a judgment regarding sexual appeal pretty much demonstrated that respect was not part of the equation.
I should also note at this point that those of you who felt you agreed with me on this issue probably don't want to actually GO to that link. Just trust me that I'm conveying the information accurately.....
But Wiki didn't have the history. It did mention that "I'd hit it" and "I'd tap it" are synonyms.
Next I stumbled on a site that has lots of "hit it" and "tap it" pictures. I'm sorry I found this site, not just because I had to look at more "hit it" pictures, but I thought freepers were making up all these clever original pictures and it turns out a lot of them are just lifted from another web site. What a dissappointment, like finding out there is no tooth fairy. (sorry to those who didn't know that already).
Next I found an alternative Wiki site, which looked promising, giving a history of I'd hit it, which I excerpt here: Nobody knows the true identity of the person/people who originated the saying. All we know for sure is it was said by somebody who really didn't have any chance of hitting it.
Needless to say, that was dissappointing, but it turns out I think the guy was just making things up for fun, as part of his history states as fact that the first user was some guy in Cuba who was misinterpreted, thus leading to the Spanish-American War and thus leading to some confusion as to the saying's meaning. You might want to check it out anyway, he has a theory that involves Oscar Wilde and Joan Crawford that might be worth persuing
Running out of patience, I decided to bias my research by asking for both "I'd hit it" and "Ike Turner", which lead to a lot of entries for Tina Turner, who was a singer that apparently Ike Turned would "hit", but not in the good objectivized demeaning way, but in the bad objectivized demeaning way of actually hitting it.
But the only reference I could find to the words "I'd hit it" and "Ike Turner" was on a blog that had a picture of Brittney Spears with a shaved head, where a commenter said "As long as Brandon will continue to put up stories of Brit, I will continue to post "I'd hit it, and by hit it, I don't mean in an Ike Turner way"."
Apparently, the poster had a thing for bald women.
So, in the end my search for the true origins of "I'd hit it" were unsuccessful. If I had any gansta friends, I might have had more luck.