Right. It seems like the biggest distinction is it being "sexual". Complimenting someone is fine, but the second it becomes about my BODY then it's a new level.
Taking someone's photo and saying "Wow, beautiful" or "Your tattoos are awesome." is very different than "Your tattoos are sexy." or anything that suggests sex is being thought about rather than a human just appreciating another human. I suppose this language can differ from person to person. So that's where the ultimate struggle is, but it's safe to say that anything even mildly suggesting something sexual is usually when people get nervous.