Marriage Line is also called Line of Affection/Line of Sex. 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.