In particular German native speakers seem to have trouble correctly using the words pair and couple when referring to two people (married, or otherwise romantically involved), most likely because both words translate to the German Paar. In fact, the difference between pair and couple is quite subtle. From the Oxford English Dictionary, we have
pair: two people who are doing something together or who have a particular relationship (for example, a pair of students working on a project together)
couple: two people who are seen together, especially if they are married or in a romantic or sexual relationship
While both words can refer to two people who have something to do with each other, only couple is commonly used to describe a romantic relationship. Hence, in the following examples, couple is to be preferred over pair:
The two have been a
pair couple since high school.
pairs couples fight more frequently.
We only have
pairs couples among our friends.
We have been going to
pair’s couple‘s therapy for quite some time.