ratbastid and I generally know who each other's crushes are, and we have no problem taking out our sexual frustrations on each other
I can't remember what tv show I heard this on, but someone once said "no harm in building up an appetite somewhere else as long as you eat at home."
I think a healthy relationship has to allow for the continued humanity of the people involved. Just because you committed to one person doesn't mean you're going to magically lose all attraction to other people. What it means is that you promise not to act on that attraction. Now, people are more or less comfortable with the
evidence that their partner is attracted to other people. For instance, ratbastid and I have no problem with each other flirting with other people (then again, we didn't have a problem with each other SLEEPING with other people, so we're probably not the best example) but it's generally rude to ogle people of the opposite sex in front of your significant other, unless you know for sure the other person won't mind (ratbastid and I think it's cute when one of us does a double take at a pretty girl).