Not asking first
Consent matters whether it’s in person or over the Internet. Don’t forget that some states are cracking down on what they call “cyber flashing (though some, sadly, are not criminalizing it).” If you’re a woman, you may think that men would welcome surprise cybersex, but there is no harm in asking, and there can be a lot of harm (like legal damages) in not. Plus, it’s sexy to lead into it. You can chat playfully about the possibility of taking the conversation in that direction before actually disrobing or touching yourself, and that conversation alone can heat things up. However you get into it, just make sure you’ve received consent first.