Earlier quoted context omitted.
It could be. There's no context here. In this constructed example sure, it's not a friendly gesture. But it is a poor example. Here's the one I'd use: "Hey employee #333 want to sex" See? No context necessary
I agree that there isn't enough context. The example doesn't even specify that it's being sent to a female employee (that's an assumption everyone is making) and I can imagine plenty of scenarios - with context - where it wouldn't necessarily be inappropriate.
They make it clear that it was sent to a woman, and that the scenario at the very least was interpreted by the recipient as inappropriate.
Yes, there are scenarios where it could be acceptable, but consider that they are (much) narrower than you might think:
* Men can be sexually harassed too.
* Sexual harassment is not the only problem with an e-mail like that. It may also create drinking pressure, or a feeling that you are forced to socialise with your boss. Of course that is context-dependent too, but for my part, as a man who doesn't enjoy drinking and doesn't want to spend my Saturdays with the boss, I'd fell put under inappropriate pressure by an e-mail like that as well, because I'd be concerned that a rejection could impact work.
A lot of behaviour like that is fine when it is not between subordinates, and can be fine with subordinates too if it is someone you know very well.
But even then you need to consider that e.g. it may negatively affect team dynamics or may be unintentionally discriminatory or become seen as discriminatory purely by e.g. including only those the manager knows well too.