Lots being said about "making sure the wife understands." But little or nothing has been said about you. Consider: you are going to be gone for 3 or 4 days at a time. You are going to be in close quarters with a lot of females: flight attendants, gate agents, crew schedulers, fellow pilots, etc. These are people who have chosen a career and lifestyle with a lot similarities to yours. Which means you have something in common, something shared that draws you together. A good portion of them are likely to be attractive. There are a lot of jokes about it, but in my experience, the airline business seems to attract a sizable share of good looking women to it.
Now consider this: You are on day 4 of a 5 day. You're tired, you're homesick, maybe it has not been the smoothest of trips, so you are a little depressed or frustatrated. And now that hottie flight attendant you have been flying with all month is letting it be known that she is available to you if you are interested. What are you going to do? Are you strong enough?
Consider your own strengths, not just your wife's. If there is infidelity in the marriage is just as likely to come from the person on the road as it is from the person left behind.