If hitting on a coworker were illegal sexism, a good part of the slashdot audience wouldn't be here, because their parents never would have hooked up.
It's not appropriate for someone to send messages like this to a subordinate, period, the end. It creates a hostile work environment because they have to worry about whether they'll be penalized for saying no.
The appropriate response to someone walking in with a fistful of evidence that someone is engaging in sexual harassment is to fire the harasser, immediately. This is especially true anywhere that has had sexual harassment training. And basically all tech companies are doing that now, and this sort of thing is evidence that it is necessary; both the event, and all the jerkoffs scrambling to defend what is clearly unacceptable behavior.
Uber has a rule against sex between drivers and riders, no matter what. I guarantee you that their employee code of conduct bans sexual harassment, and clear sexual advances like these without invitation are a clear case of sexual harassment.
Finally, it wasn't actually his first offense, that was just a lie told by HR. Because HR is not your friend. Get that part straight right now. They work for the company and their job is to smooth the rough, pacify the angry, and meet legal requirements. It is not to help you.