You are confused about marriage line as they are also found on hand of saints and prostitutes.
Marriage Line is also called Line of Affection or Line of Union. Marriage line does not necessarily show marriage it may only indicate attachment to someone male or female. In other words this line can indicate anyone whom you consider a friend or loved one.
Marriage line is often found on porn star, prostitutes, unmarried person, celibates, saints (sadhu), and kinnar, bisexual, hinjda, eunuchs, and in such cases indicates strong friendship.
So every marriage line does not necessarily represent marriage but it does represent an affection that you have felt or feel for someone.