these threads are interesting as people (mostly) say they could never be in a relationship with someone who was unfaithful or if they didn't trust them, yet so many men seem to stray. I dunno. After being a family lawyer for several years I guess I heard "I never suspected a thing" too many times.
I think if you are in a relationship that is respectful and meets the needs of each other at that time, or if it doesn't one party can empathise for the reasons why, then yes you should expect your partner to be faithful (if that is important to you).
However, if you treat your partner badly, or ignore your relationship, it might be too much to expect. I'm not excusing the behaviour; it's just what my experience has shown me.