Cheating at company picnic

We just had our company picnic and two of our married employees were cheating with each other (they are married to other people...). We work for a small comany and this is really an issue. Can we fire them?
Comments
We had a married manager (male) make out with an unmarried employee (female) from another department right in front of everyone at the picnic. It certainly looked consensual. I think alcohol might have been involved.
We are a small, family-oriented company with at-will employees. We don't have a policy on employee dating.
The owner fired the manager overnight. The embarrassed female stayed a month or two and then resigned.
I think most employees admired the owner for upholding values.