Look on the top bar where all the links are. You'll see one called "Profile". Hover over that and click on "Account Settings".
Once that screen appears, look near the top and you'll see a link called "Modify Profile". Hover over that, and at the bottom of the list you'll see an option called "Group Membership". Click on that and it will show you a list of all the Membergroups you're a member of, and all the Membergroups you're eligible to join.
You should have the option next to the "MTF Transsexual" group to Leave Group. As far as I remember, 15 posts is the minimum required to edit your profile information. If that's been changed in my absence, I am sure one of the mods or admin can do this for you.