My officemate got me interested in this question, so I'm putting this here for people to answer.
If you were in a committed relationship and your partner admitted to sleeping with someone else, would you be more offended by:
Your partner having an affair (not paid for).
Your partner hired a prostitute.
You wouldn't be offended.
In the above scenario, if you were the one having the affair, do you think your partner would be more offended by:
You having an affair (not paid for).
You hiring a prostitute.
Your partner wouldn't be offended.