is
eg.a couple is going to meet up at the movies.
一对情侣要去电影院相会
A couple is making dinner together.
一对夫妻正一起作晚饭.
are
eg.A couple are leaving for work in the morning.
一对夫妇早上正准备离家去上班.
Tracy and Jason,a couple,are talking about their family financial planning.
蕾西和贾森夫妇俩正在讨论他们的家庭理财计划.
查了下牛津词典 上面是这样表示
[C+sing./pl.v.]
eg.The couple was/were married in 1976.
我想,如果是a couple ,用is 更规范些吧