No, not everyone cheats; however, as I said in another thread most people would given the right set of circumstances and person even though most of them would tell you otherwise. Get them behind closed doors with an understanding of discretion and privacy and if they are feeling the person enough that shyt goes right out the window.
Not all women are cheaters just like not all men are either, but I would venture to say many probably are cheaters, I would venture to even say much more than half of men and women have, will, or are going to cheat at some point.
Women cheat just as much as men do as well. Women just don't do bone headed shyt to get themselves caught like leaving information in cellphones for prying eyes to see or have their sidepiece calling the person they're in a relationship with to start drama. Most women don't operate that sloppy with their philandering unless it's already understood she's promiscuous.
Recently as a week or two ago I saw a guy I know on Facebook talking about how some men are ducks and their lady is messing around behind their back, when I know for a fact his wife was messing around behind his back and he didn't know it.
Most people men and women are about as faithful as their options. The more success, money, and popularity a person has the more options they will have too. The person's level of physical attractiveness obviously plays a big part too; however, in most cases even lesser attractive and less successful people have options albeit to a smaller degree. That shyt is a lot to overcome for people that are not extremely disciplined, content with what they have at home, and not easily enticed by the idea of something new and unchartered.