Ya know, it can be very dangerous for women to reject a man. He might get angry, beat you, rape you, kill you, for the audacity to not get enamored by him at first glance.
One of the easiest ways to get a guy to stop pestering you is, sadly, to tell him you have a boyfriend/husband. Funnily enough, that deters most men, but not always for your sake. Sometimes it's for the OWNER's sake. It's "Ah, someone has already claimed her; can't violate HIS rights to HIS property, can I?" And most of us women don't like to use this way of rejection; we don't like to be seen as property. We'd rather the guy took a simple "No thank you, I'm not interested" as an answer.
I used to work at McDonald's when I was younger. On late Friday and Saturday nights, quite a few desperate guys grabbed a burger on the way home, with no conquest for the evening. Some of them tried a last stab; chatting up a tired, sweaty and fry-grease-smelling young woman behind the counter, who was obliged to be, if not kind, so at least polite to them, by her employer. A pro-tip; you do NOT get flattered in that situation if a clearly desperate guy is hitting on you. Once, when I told a guy in a similar situation that I had not just a boyfriend but a husband, the desperado said "He doesn't have to know" or "He'll never know"; something like that. In my head I was like "Hey, asshole, I WILL KNOW. What about ME?".
In short, "I have a boyfriend" is the least "burtal" rejection you'll ever get.