There are actually several reasons as to why we give people the option to disclose vs not disclose their gender. First off, not everyone wants to communicate it. They want it private for personal reasons. Another has to do with people not always identifying with what they are physically. And still another is because there are people out there who aren't solely one or the other and thus making a choice between genders just isn't possible. Given this, we feel it best to let people have the option of choosing whether or not they wish to disclose such information.
We do understand that some people have issues RPing with certain genders, but given that this is an adult site, it's our hope that people will act as such and respect such things of its members. For instance, you aren't comfortable RPing with males and there's a male who won't disclose his gender and RPs as a female. The hope is that they won't seek you out knowing your preference and, if sought out by you, will politely decline. Yes, this is an honor system of sorts, but we trust that our members won't maliciously try to dupe others.
I hope this answers your question. Thanks.