Both "married couple" and "married couples" are correct, but they are used in different contexts.
"Married couple" (singular): This refers to one specific pair of individuals who are married to each other. For example:
- "John and Mary are a married couple."
- "A married couple lives next door."
"Married couples" (plural): This refers to multiple pairs of individuals who are married to each other. For example:
- "The event is open to all married couples."
- "The workshop is designed to help married couples improve their communication."
So, whether to use "married couple" or "married couples" depends on whether you are talking about one specific pair or multiple pairs of married individuals.