Number of Google Results
Postgender- 1,004,100 Transhuman- 841,000
Postgenderism- 77,000 Tranhumanism- 1,340,000
Google scholar
Postgender- 1,330 Transhuman- 4,720
Postgenderism- 10 Transhumanism- 1,910
So even though people considering postgenderism are apparently a slightly smaller minority than transhumanists, it's not a completely undiscussed topic: Both topics are being discussed both in scholarly and in non-scholarly modes.
I can see how you would assume that because the transhuman community doesn't do much discussing of postgenderism, that it must not be that popular, but in fact it is just that they are pulling from a different pool of adherents. Postgenderism has its roots in feminism, masculinism and transgender movements.
Note that this may be an unrepresentative sample. Both non-standard gendered individuals and transhumanists are often groups considered to have disproportionate internet footprint and compared to their actual size. Given that, using this data to decide that one minority is "slightly smaller" than the other seems dubious. Similar remarks may apply to the academic footprint (although my impression is that postgenderism and related ideas are much more common in academia than transhumanism.)
Upon reading Eliezer's possible gender dystopias ([catgirls](http://lesswrong.com/lw/xt/interpersonal_entanglement/), and [verthandi](http://lesswrong.com/lw/xu/failed_utopia_42/) and the other LW comments and posts on the subject of future gender relations, I came to a rather different conclusion than the ones I've seen espoused here. After searching around the internet a bit, I discovered that my ideas tend to fall under the general category of "postgenderism", and I am wondering what my fellow LessWrongians think of it.
This can generally be broken down to the following claims:
EDIT- Due to some really insightful comments;
I replaced men being prone to aggression as a negative, with men being prone to suicide.
I made the verbiage a little more explicit that no one would be *forced* to change, but would seek out the changes that transhumanism would have available.