marriage partner; spouse
Refers to the person one is married to, regardless of gender.
It's important to share values with your marriage partner.
future spouse; fiancé(e); husband/wife-to-be
Used for someone one is engaged or planning to marry; context determines gender.
彼は結婚相手を両親に紹介した。
He introduced his future spouse to his parents.
She has been looking for a marriage partner for a long time.
He chose his childhood friend as his marriage partner.
After starting matchmaking activities, I finally found a marriage partner.
I think he's ideal as a marriage partner.