It's nothing new. I have been online for years and always had to deal with that. I have a plan: If they bring up sex in first convo I give them a warning. If they keep it up, I ditch them since they obviously are not the person I am looking for. The more you nip it in the bud the easier it gets. Don't feel bad about not messaging them back. You are your first priority. I believe in being courteous but if they show they can't be, then I can't either.