You shouldn't even do that because there are plenty of dudes out there that use a female avatar
Chat can be easily monitored and limited though, plenty of MMOs out there that just blocks inappropriate words and problems there isn't too big of a deal.
And they could easily add a report button and have people banned if necessary.
They could also just make chat a feature between trainers in gyms because while you do have to go to the gym to interact with it, chances are you won't run into the other people that adds their Pokemon in that gym and it would just be a nice to be able to leave a message there for those helping you hold down the fort.