Coming from a guy, and take no offense in this, it comes down to two things. 1. How you present yourself and 2. Odds.
Again, I've never seen your profile so I'm speaking generally. I run into quite a few profiles on various sites and the honest truth is some of the girls look like I can say a few lines and throw a few dollars at them and get them in bed. These are often the women who have a bunch of bikini or "sexy" pictures and mention that they always down to have "fun". Sometimes even the username give it away.

On the other side, it's all about the odds. When you're REALLY trying to win the lotto, you don't buy a single ticket, you buy multiple tickets. The same thing applies here. Throw a cheesey pick-up line at 15 people 3 may take the bait, as time goes 2 may stop responding for one reason or another and the last may agree to meet up.

So with those 2 things in mind, that's why those guys do what they do. It's just the way these dating sites go. The best thing you can do is ensure your profile doesn't promote this and when you are approached, simply delete the message. Don't even give it the time of day to reply. Also keep in mind that because most of these guys are likely using a message template you're still likely going to get these advances, it's just how it goes.
As a guy, I get messages from time to time about watching "a girl" on webcam, visiting a gentlemen's club, or registering for a webcam site. It's just the nature of the beast. Weed all that crap out and look for your diamond in the rough. Whether you know it or not, there are guys out there doing the same as you - truly looking for something real.