So, a couple of things. How old are you? I suppose it isn't really that relevent though.
It's perfectly normal to be aroused watching men do their thing, just as it is for men to like watching women doing theirs.
Everyone wants to share their sexuality with someone else, and if you're single then a webcam is a good way to do so...
Other girls certainly do do this, although not enough .
Get yourself some sex toys or a boyfriend and roll with the fun.