Is a workplace responsible for enforcing no coworker affairs with married employees?

No it is not, the workers themselves are responsible for their own emotions and desires. If affairs do take place it is because those involved want them to happen. Some workers are attracted towards each other and this is fine if they are single or unattached. To be unfaithful to ones spouse is a deliberate act of adultery, but one cannot blame the workplace for this affair. The blame lies in a persons lack of self control.