Amy, this is a hard question to answer. Because it depends on the situation, one has been through, etc.
Do all men cheat? That is a hard question to answer. I think that men and women, both, cheat. I have seen the other side of the coin, where the woman was all about dating and doing it just for sex. The reality is, it is out there and ppl out there like that.
Now you really are asking can a man and woman stay faithful to one another? That is another hard question. I have been married twice, and I am here to tell you that marriage is a JOB, 24/7, 365 days of the year one at that. Once you start slacking, on either side, it really starts a demise that you both have to overcome. And this is not about sex, but about the "togetherness" of marriage or any long standing committed relationship.
Now, about the sexual part of your relationship. I do not know what it is that you will or will not do, but have you both thought about counseling? Obviously, he needs it and it could be an eye opener for you too. But I do know this, if you are uncomfortable in any aspect of sex, and if you believe/feel it is not giving you any pleasure then do not do it. If your ex, is really perverted, then do not shed anymore tears "boo-hooing" over him because he wants you to perform some crazy antic and takes no consideration of your feelings. Overall, he sounds and reads like a jack, and time for you to move forward.
Finally, if you are really shaken up about you sexual being, there is nothing wrong in counseling. Because, I think this has a lot more to do with you being in a relationship versus the sex. I can be wrong, but that is my gut feeling. Hope this helps.