^ I think it's pretty even. Studies have suggested that men cheat marginally (key word marginally) more than women and from my own personal experience I'd tend to agree with that.
My personal views on cheating is that it need not exist in my life, at this point in time. Talking concepts of exclusive relationships, open-relationships are the way to go. Why bother yourself with the stress of whether your partner is cheating on you or if they would, when you could be having great pillow talk about fantasies you both have re: other people?
I hate wondering, being in the lurch is one thing my head doesn't cope well with - at the same time I'm a very accepting person... if I know what's going on, I can cope with just about anything. That includes my s/o being with other people... infact, I generally like the idea and it plays a role in my personal fantasies on a regular basis.
I don't think cheating is acceptable - I think we should all be in relationships suited to our needs. Obviously if you're having to break some sort of moral responsibilty by cheating, that relationship isn't suited to you at that point in your life... Still (as SLM would say), the 1 chance rule is probably a safe rule of thumb to stick by (assuming the relationship is otherwise healthy). People do make mistakes and they can be just that... mistakes. If you've got a *really* good thing happening, it makes no sense to me to throw it all away because of the 1 mistake that is most likely deeply regretted.
If it happened recurringly, the cheater needs to be involved in a relationship that's fair and just on their partner. A type of relationship that allows them to do their thing, yet allows their partner an equal ammount 'freedom' to be with others, so to speak! All relationships have a set of agreed upon 'rules', some relationships just have more rules than others...
I've spoken to a lot of people who are in exclusive relationships who cheat and asked them why they didn't go for something where they could be fairer on their partners? I seem to get a response along the lines of "I'm too jelous a person to acknowedge that someone I care for is also with other people". Sad but true!

Too many members of society have one set of rules for themselves, yet another for their partners.
I think it stems back more to fear and feeling inadequate. Have they actually given something new a shot? Searched well and beyond their comfort zone? no... (not the people I've spoken to anyhow).
I was once like this too - could get overwhelmingly jelous at the drop of a hat! I didn't like that about me, I hated it... I forced myself to push my own boundaries, try new things (no matter how uncomfortable) and I *shocked* myself. I realised that all the fear lay within my head and once something actually happened, it wasn't bad at all - like I had expected it to be.
My own head, for a few years, dominated who I really was as a person. Tried to tell me what I'd hate, without even experiencing those things. Since taking the chance and stepping outside my comfort zones, I've discovered a lot more about myself as a person. As a result, I am *genuinely* a lot better off. Word of advice: Try new things people, even if you don't think you'll like them. 2 years ago you wouldn't have caught me saying a word of this - now I'm reccomending it!