You cant generalise, every one is different, i agree about not having the full trust for someone once they have cheated on you.
I know a couple that have been married for 30 years and both have had affairs once, they are still together and are happy and regret what they both did and would never do it again, it actually made them stronger.
On the other hand there are some men and women who just for some reason continue to do it, to me i think its becuase they are insecure, or simply just cant be faithful to one person, so to be honest i think it depends who the person is.
I have been cheated on by two different partners so can look at this in two lights, you do lose the trust but you can build it back up but only you can do that, you cant let it make you bitter and twisted or be revengful.
So i would say once a cheater not always a cheater as it depends.