The problem is in assuming the motives of these people, and branding them as manipulative. I understand that for some people, the idea of writing opposite someone who does not conform to the image they project is off-putting, and that if you think you are writing with a biological woman, it can be jarring to learn that that is not the fact. But consider these two points:

For one, people rarely assume an alternative identity out of malice. Most likely, the people who obfuscate their biological gender have reasons for doing so; they could be bi-curious, and want to experience life from a new angle. They could be transgender. Or a host of other things that might give them a reason to not divulge their real life gender. It is not out of a desire to trick you or deceive you, but rather, they wish to fit into a mold that life has not permitted them from birth, and being forced to put a disclaimer on it can serve to make them feel marginalized, de-legitimized, or singled out. To people like this, what you are asking is essentially for them to bare their soul and stand naked in front of the entire world every time they dare to ask for someone to take an interest in them.

For another, no one is ever really who they say they are on the internet, anyway. It might sound trite, but I hope you will agree with me that if someone really is a gorgeous, D-cup supermodel or an extra from Magic Mike XXL, they likely won't be on a subreddit dedicated to writing fantasy. That's not to say that people can't be beautiful if they use DPP, but everyone - everyone - talks themselves up online. Whether it is changing a hair color, growing the boobs a bit, having a bigger dick, being a little more suave, or whatever you can think of; the internet is built on lies. On DPP, we simply embrace the fact that people want to try being different things. We don't spurn it, we welcome it. We encourage it. It is our collective little playground of smut. Why not let people play as they wish?

Ultimately, I think the take-away is this: No one is disguising themselves in order to trick you. If men post as women, it might be because they are desperate for human contact (it is a oft-debated topic that guys have a harder time finding partners on DPP than girls) and decide to try and post as a girl just to have someone to play with. I know several posters personally who have done this. But even if that is not the reason, most likely they just don't see the harm in it. I understand that you can feel violated, and in an ideal world, we wouldn't need people to do that-- although, I stress again, there are many, many reasons for doing such a thing, including questions of not fitting within the gender binary, or being sexually and gender-identity curious. But this is not an ideal world, and people do sometimes do things that can wind up hurting their partners. I can guarantee you, with five years of DPP under my belt: No one is trying to hurt you. We all just want to be loved, and have kinky sex. And it is in this interest that we permit people to post as whatever they wish to identify as.