Definitely not unrealistic, with regard to the original post.
Yes, everyone's human and plenty of decent people are tempted elsewhere for whatever reasons. I think though that as a reasonable adult, that person should discuss their feelings with their spouse and see if they can address whatever the issues are. If they can't - if it's a matter of simply wanting someone else - then they owe it to their spouse to give them the choice of whether that's a dealbreaker for them.
This is my issue with cheating... not people sleeping with someone other than their spouse, but the complete disrespect shown to not give the other person a choice as to whether they want to continue in a relationship like that.
Of course, when people cheat, they generally don't think they're going to get caught. Have your cake & eat it too, right? You can get what you want, and not lose face or your relationship.
So for me, DP sleeping with someone else wouldn't necessarily be a dealbreaker. I don't know - it might be, but I couldn't tell you unless it happened. However, cheating on me and keeping it from me....yeah, that probably would be. I wouldn't want to lose our relationship, and I'd consider counseling etc., but I don't know that I could get over that kind of dishonesty.