Would it be wrong for me to sleep with someone who's been in a 5 year long, monogamous relationship?
What makes you ask that kind of question?
The opportunity kind of presented itself and I'm thinking of going through with it.
Well, I think it's great that you at least thought to get advice first but, sleeping with someone who's already in an exclusive relationship is wrong. The person who propositioned you should be ashamed.
I don't understand why that would be so bad.
Well, it's wrong because of all the problems it can cause, for you, for that person and that person's partner. They're in a committed relationship so sleeping with you would be cheating. They aren't happy with their partner about something, and they should figure that out and if their partner finds out it just adds even more problems.