Its all fantasy, really. I mean, when you boil it down, just about any fantasy involves objectification. You're reducing a person, or a type of person, into an object for fulfilling a fantasy. Its especially prevalent in porn, because when you get down to it, porn isn't about 'people', its about fetishes, and the people serve only to fulfill those fetishes. Big breasted women, flat chested women, black men, Asian girls, lesbians, gay men, and then further down to the various 'types' of lesbians and gay men; butch lesbians, lipstick lesbians, twinks, bears, etc. Even if the porn itself isn't degrading or dehumanizing, the point is still to use the person or people involved as a sexual object to fulfill a fantasy. I have never seen any porn and thought to myself, "Wow, that person has some great personality." Its not that porn stars aren't people, I've seen some porn stars on the internet who seem to be really cool people with deep and interesting personalities. In fact, I used to know a girl who does porn, and she was a perfectly rich and fulfilling person to talk to. Its simply that, while watching porn, I never found myself thinking, or even caring, what they were like in person. They were acting as the object of a sexual fantasy, and that was the point of it all, and in many cases its less about the person and more about the 'type' of person that they represent.
Even when you're not dealing with porn, the same can often apply, as long as the ultimate goal of the work is less about telling a meaningful story, and more about fulfilling some romantic fantasy, or even a sexual one; simply because there's no sex in the story doesn't mean it can't be used to invoke sexual fantasy, merely by implying that sex happens off screen/panel.
I'm not saying reinforcing these stereotypes is a good thing, but its not especially bad either. As long as the viewer understands the difference between the stereotyped fantasies in the fiction, and the fact that the people in real life; even the very people portraying these stereotypes, if the medium is live action; are more than that. It's sort of the same as people who enjoy various fetishes which may not be acceptable to do in real life. As long as they understand that the fantasy is different from reality, I see no harm in it. Even if there is harm in it, unfortunately, you'll have a hard time trying to change people's sexual fantasies. On the bright side, I think anyone who isn't severely mentally ill can understand that porn is pretty much all fantasy, with very little reality involved.
Now, when we're talking about more mainstream works, I think it becomes more important to try and change the perspective because these are SUPPOSED to represent our cultural identity, and if all we do is portray stereotypes, that's what people will assume is reality, if they don't have an actual basis in reality to work from.
And as A Japanese Dane said, it does vary from the author to author. I'm not big on yaoi or yuri manga or even straight romance manga for that matter. I do find a lot of it, not just yaoi, is stereotyped and characters tend to be fit into very neat little holes. "This is the childhood best friend who's a bit of a tsundere, this is the mature teacher lady who's unusually sexy for her age, this is the little sister with a crush on onii-san." As A Japanese Dane also said, its very much an aspect of Japanese culture. I imagine it comes from the more Asian emphasis on conformity, where we in the west have a near fetishistic love for independence and originality.
Perhaps its just a matter of values dissonance over a cultural divide, but I find that the best romance in any manga or anime are those that dare to break the mold. For example, if you want my all time favorite romance manga. (Warning: Is a Yuri manga, contains nudity and even some sex, though it tends to focus more on the characters and the story.) Hanjuku Joshi, or almost anything else by Morishima Akiko is absolutely amazing. She loves toying around with the cultural expectations of her characters; including the idea that one has to be the 'man' in the relationship, the idea that schoolgirl romance is a passing fantasy and they'll grow up and get men when they're adults, and most notably in her work, the idea (which is admittedly, from what I understand, beginning to become less common) that a woman is no longer desirable after she's hit 25.
I'm not saying that her work is perfect and without stereotype, but it really likes to play around with stereotypes, and even break them completely at times. Add to that, the fact that her characters are well-written and feel like real people as opposed to being fantasy objects, and her work tends to focus more on drama and romance than 'and then they banged'.
I admit, I can't draw any good examples from yaoi itself off hand, but I'm certain they exist xDhttp://tvtropes.org/pmwiki/pmwiki.php/Manga/HanjukuJoshi
Here, if you want a work safe way to see what I was talking about with Hanjuko Joshi xD