I clarified that I did not mean to imply that unbelievers are not able to enjoy marriage ... did you see that clarification at the bottom of the post?
As far as your statement "there's no guarantee that it'd be enjoyable for Christians, nor is there any guarantee that there is a happy marriage reserved for every good Christian in the first place" ... I never said life was not without it's ups and downs.
I know about the peaks and valleys. If and when both husband and wife love God with all their heart, soul, mind, strength, the husband and the wife each help the other to remain focused on the things above ... those things which are eternal. The husband and the wife encourage each other when times get tough ... help each other to stand strong in the Lord.
What believers have together that unbelievers do not have are the eternal realities and truths. Unbelievers have no hope as do the believing couple. Other than worldly wisdom and philosophy, unbelieving couples have no source to rely on when the going gets tough as do the believing couple who look to God's Word.
The rough times last for a season and then there is relative calm which the believing husband and wife also enjoy together, having been perfected, established, strengthened, settled as God works within during those times of suffering (
1 Peter 5:6-11).