When you click the gray flag (top right next to time stamp), there is a Report Reason called "No Sexuality Topic". This is to be used when something is not correctly in a Sexuality topic AND when something is incorrectly in the Sexuality topic. Reporting helps us get to the quicker, but it can still take up to 24 hours before we're able to get to it and correct it. We are working hard to correct the confusion, sending reminders every time a user uses the wrong topic. Over time, everyone will learn.
Questions about penis size should always be reported using reason "Spamming" as we don't allow penis size questions, since the site is already flooded with every possible question about it.
You're right, there used to be a "Wrong category" option to report, but not anymore. Now everything is antagonistic instead for some reason. This would be useful for questions that are in "GirlsAskGuys" topic but are actually about body ratings and unrequited love.