OrphanPip & Gizzie :o . lol
I kind of already knew about the play on the parody of M/F relationships in Yaoi (and sometimes Yuri), and that's what I think the point of it was in the beginning.
Let me explain.
From what I've read, Yaoi was originally created, by women(straight usually), in a parody of Heterosexuality, especially of the "straight" animes. Not to be funny, but to shed light on a topic/issue, that not many people were very aware or were reluctant to be aware of. I'm not Japanese (I'll leave that to Gizze :p ), but I was always under the impression that, in a tradional family, the sons were to marry and carry on the family name and that homosexuality was forbidden, especially in rich families. This is probably not how it actually is/was, but if it was, than it makes sense, because otherwise, a very Noble bloodline would die with the last son, who is homosexual, unless he marries and has a son himself.
I also think that, because men were reluctant(and still are globally) to be expressive, especially about sexuality (especially in Japan, as Gizzie says), Women did. Women being naturally expressive, would of course over dramaticize M/M relationships, because how they see men & gay men especially, is different from how a man sees himself and other men, gay or straight. The opposite gender always views each other differently, because we obviously don't have insight personally on how the other works on a systemic level.
This is why a good deal of straight men believe lesbians are "cock-hungry nymphos" that can't wait to get it on with each other and then when he introduces himself to the equation, they'll jump on his dick. This is how a man thinks, and because he thinks that way, that's how it must be, taking into account nothing of how an actual lesbian relationship works, sex or no. Similar for women, they believe all feminine gay men are flaming queens that going shopping with them for Prada handbags and are bottoms, just because you look like a duck, quack like a duck, so we must be a duck.
A woman is going to write her point of view on how a M/M relationship works, based on how it would work with her, so yes, it's a slight inversion. But they don't think the same way a man would, let alone a gay man, so as a woman, she's going to be penetrated, therefore, whoever she writes as "the woman" will also be the one to be penetrated (Uke). Again, since she doesn't think like a man or understand personally how men work, she's going to write "the man" as the stereotypical "male" with attributes, equivalated to being a man.(Seme)
I personally find nothing wrong with Gender Roles, but I don't think it's like the 1800's when women were barefoot and pregnant at the stove and men were out making the money for the house and family, Gender roles. People assume that, just because your flaming hot, that you're the bottom or if you lift 200lbs weights and chew iron nails, you're the top. It's not always this way. I think the problem people have with gender roles, is that they are afraid of people catorgorizing them into a specific role in the relationship, or just in general, that they don't like or want to be in, which is fine. But Yaoi doesn't necessarily limit a character to a specific role, but rather shows diversity within the character's persona.
Example; The Yaoi Sukisho(that I posted earlier) has a butch/femme theme. Simple? No. The Feminine male in this Yaoi has long hair, that's even pink("girl colour") and has a soft voice, but isn't very "feminine" outside the bedroom. The other main male character, who's the lover of the feminine male, is portrayed as a typical male, with short hair, that's blue("boy colour") and is very "normal" . Now their alter ego's kind of mess with their personalities, but the pink haired one is very vocal and domineering, while the blue haired one is also vocal, but very submissive when the pink haired one becomes aggressive.
But their alter egos are complete mirror reflections. The Pink haired one becomes womanly and the blue haired one becomes even more masculine, even so much that his voice deepens.
I think this shows the mix of masculinity and feminity in all of us, rather than just the inversion or parody of heterosexuality. Feminism and Masculinism is a whole-nother ball game, but people try to connect it to these Yaoi, when in actuality, it has nothing to do with it really. So one is always the "girl" or the guy? It doesn't mean that all gay men have to be this way. I've seen Yaoi where some guys are Versatile and not always in a specific gender role.
I think that a play on sexual or normal gender roles in not an inversion, but rather a progression. And after all, we all know who we are, what we like and how we want to live our lives and perceive/be perceived by others, so it shouldn't matter how some choose to express themselves.
Love you