There are many beautiful vocations in life; priesthood, religious life, serving God in some way as a single person, and yes, marriage. If a person has it in their head that it is inevitable that they will get married, that their future husband or wife is out there somewhere waiting for them, they may miss out on the special thing that God is calling them to. If they find themselves at a certain point in their life still unmarried with no serious contenders for a spouse, this could lead to anger, depression, and a questioning if God is really there for them. "I've been praying for my future spouse my whole life God, why aren't I married yet!?" Perhaps the answer is that you are meant to be a bride of Christ, but you just haven't been listening.
So rather than praying for future husbands and wives who we may or may not meet, I submit that we should pray to have open hearts and minds ready to listen to God's will, and to have the strength to be willing to do whatever it is that God asks of us. For many, that may be marriage. For others, God may take us down a different path. Each vocation has a special and unique gift to offer to the world and to God. If all people were open to what God was calling them to, just imagine what a wonderful place this world would be!
Prayer to Know One's Vocation (Carmelite Sisters of the Divine Heart of Jesus)
Lord, my God and my loving Father
You have made me to know you, to love you, and to serve you,
and thereby to find and fulfill myself.
I know that you are in all things,
and that every path can lead me to you
But of them all, there is one especially
By which you want me to come to you.
Since I will do what you want of me,
I pray you, send your holy spirit to me: into my mind,
to show me what you want of me; into my heart,
to give me the determination to do it,
and do it with all my love, with all my mind,
and with all my strength right to the end. Amen.