The reason being that Marvel is trying to target the female demographic more, and epically failed at that when they put Spider Woman in a pose where she looks like she's waiting to get f**ked from behind, whereas Spider Man looks like he's actually crawling like a spider. I don't know what made them think it was a good idea to hire Milo Manara for that job, considering the kind of work he's done before. This is actually a prime example of why more women are needed in the big companies...in general, women are more likely to understand why the Spider Woman cover is sexist, hence a woman creator would more likely make something that doesn't alienate female readers...
I mean...look at that....what were they thinking!? Milo Manara is known for making pornographic stuff, and that's it's own genre, and that's fine! But to hire him for the Spider Woman cover....I mean...there MUST have been several people seeing that cover before they decided to print it...did no one at any point stop and think "Hey...maybe we should draw her more like we draw Spider Man? We have tons of reference pictures!" ....Like the one to the left for instance. Instead of having Spider Woman on her knees, with her butt in the air, they should've gone with the Spider Man pose...his whole body seems almost glued to the ground, there's nothing sexual there. It's so bizarre, because the Spider Man image demonstrates how easily they could've avoided that whole discussion. Draw her like you'd draw Spider Man. Simple as that. Superheroes often do acrobatic stuff because...cool fighting scenes...and yeah, sometimes that might result in weird positions that can sometimes come off as sexualised. But this wasn't a random frame from the comic, this was the cover. That's how they sell their comic, that's the most important way for them to tell their potential readers what to expect to see more of inside. This just sends all the wrong signals, which the uproar is proof of. Unhappy buyers means less money, so they had to do a better job.
For the record - I don't hate Milo Manara, he's very talented, just the wrong pick for this cover. He did a wonderful job in Neil Gaiman's Endless Nights, where he illustrated the story about Lust...he was the perfect fit for that one, so if anyone's interested in seeing Milo Manara's skills utilised in the best thinkable way, definitely check that story out, the illustrations are gorgeous, and the sexual undertones (or even overtones) fit the story and the characters.
EDIT: I feel like I should add that the Spider Woman cover wasn't just about the Spider Woman cover. It was also about how this was yet another example of how female characters are often portrayed in a more sexualised way than male characters. So while it seems like a big uproar about one specific cover, I think it was more about how this was yet another example, and people are getting fed up with it, therefore some cases get a lot of attention. If this type of portrayal of female characters was a rarity, one such cover wouldn't matter. It's the bigger picture that makes what seems like a minor thing like this turn into big discussions.