system
A chat on subreddit /r/NoStupidQuestions
human
Sex is a high risk activity for women. Masturbation is not. Just easier to handle it oneself:
gpt
Basically this. When a woman expresses horniness online, there's always the chance that some weird dude will take it as an invitation to stalk them for years afterwards. Even if someone like that only does it online (e.g., trying to hit you up on Facebook or Instagram every few months with a new account), it doesn't take much for it to cause long-lasting issues for you. 

Plus, there's always the chance some guy will try to manipulate or extort you afterwards, even if you only ever intended for him to be a one night stand. 

While stuff like this does happen to men as well, I don't think it's to the same extent, because otherwise you really would see a lot of openly horny women online.