It's funny that you mention MtF being underrepresented here on Eden, because that does not strike me as true at all. From what I have seen on here, the overall percentage of people in the forums who are MtF versus those who are cisgender appears to be exactly the same as what I see out in the world.
Your misconception is probably due to the fact that there is an unusually high number of FtM men on here. Even if 1000 transmen join tomorrow, that won't make there be any less MtFs. The actual number wouldn't decrease unless they were prejudice against FtM which I don't think they are so I do not see what the problem is.
From my experience, sadly MtF have a few more hurdles than FtM in order to pass (for those who care about passing) so it is perfectly reasonable to think there may be somewhere they'd like to go without worrying about being treated differently or talked to differently, and for those whom that isn't happening to in day to day life they may opt for it to be online. The real life issues are more in depth than needs to be discussed in this thread so I am just leaving that bit as an example.
Another thing is that this is a SEX TOY site. Many MtF have not had surgery and don't do anal or like strokers so they won't have a lot of reason to come on here. There are exceptions of course but I can't tell you how many girls have told me they want toys but are waiting for their vaginas.
FtM on the other hand have the packers which hold almost universal appeal (most guys have tried one out at least) as well as the harnesses, Buck Angel and Couch Surfer DVDs, strokers for those with big enough cocks, pumps for those enlarging their cocks, and many of the men still like at least a little anal or vaginal play.