Agape love is divine.
Agape love is not dependent upon marriage.
One does not have to be married to practice and experience sacrificial, other centered, agape love, but family is one of the very best places for it to flourish. This is why God instituted the family. To grow loving, whole and productive people for His Kingdom.
Romantic love should not be "earthly" between God's people. The two loves are intertwined.
Marriage should not be "earthly". Marriage is an institution created by God. Agape love should flourish mightily in marriage, as this is God's design. God Himself said that it wasn't good for man to be alone.
If someone is immersed in practicing agape love which take sacrifice, time, energy and resources then they can experience practicing agape love. How many have given their lives over to practicing agape love, which means love that is given without necessarily getting anything on this earth back for the effort. For the average person the family is the best Christian place to learn, experience and practice true love on a personal level. The family should be the garden which real agape love grows and flourishes. From there it can be spread out to others. Usually people who know how to love others first learned love from their loving families.
Many people are broken and mixed up because they didn't receive and learn to truly love from their families as children. Our broken society today is largely due to broken, loveless families that are not putting God first. Conversely how wonderfully people often turn out when they come from stable, loving families. Families are meant to produce loving people who have learned and experienced love from childhood on up.
When people choose not to have families for selfish reasons they are missing out on one of the greatest opportunities of life to give and experience agape love, which then can be poured out on others outside of the family.
Jesus was born into a family of a husband and a wife. He even had siblings. He was brought up in the best soil this world had to offer which is the soil of a loving family. It is God's plan.
Many people stay single not because they want to practice greater agape love as a single person, but because they are too self centered to give up their independence and their resources, which must be done if you are going to marry and have a family. They are not agape loved centered and they may well never know anything except the selfish pursuit of pleasing only their self. You may have not noticed it but many these days are single because they have a self centered mindset and they are loosing life's true pursuit, which is deeply loving and also being deeply loved.
A good marriage should have everything to do with agape love.
I am not slamming marriage. I am very happy for those who have life partners.
I agree with you, that our current society generally is not encouraging family, and that there is blatant self centeredness. There is even a ME Generation.