“I like your dress!” is acceptable. “You! In the red blouse! You’re gorgeous!” is also acceptable. “I like how your dress clings to your ass, sexy!” is NOT acceptable. “You! In the red blouse! Nice tits!” is also not acceptable.
If your “compliment” refers directly to:
their breasts (or lack thereof)
how much you want to have sex with them
how you want to have sex with them
wanting to have sex with them against their will
seeing certain parts of that person’s anatomy (sexual or not) in a specific, sexual act
calling the person a name that is considered ‘degrading’ or involves profanity
‘pet names’ on a stranger
then it is not a compliment, it is catcalling, and consequently, is harassment.
So as I was saying. There’s a line. “Hello, beautiful.” is alright. But “Hello, beautiful. Those lips look perfect for sucking my cock.” has crossed that line.