A couple — two people in a romantic relationship. 'Couple' can refer to dating partners (hetero or same-sex) and is not limited to married spouses.
そのカップルはいつも手をつないで歩いている。
That couple always walks holding hands.
レストランにカップル向けの個室がある。
The restaurant has private rooms for couples.
友達がカップルで旅行に行った。
My friends went on a trip as a couple.