I would personally not tell them because not everyone likes to know what/that others think about them. But I do not see the moral issue if I don't tell them.

What if I only thought about it? Still morally reprehensible? Or only if I tell others I think about them? Then you could argue it's sexual harassment.

I've had a coworker tell me something very close to that before. I could have been morally outraged but instead I just propositioned them for a date.

Personally I think it's just embarrassing, not immoral.