If you don't have marriage line, it means that you have no desire to love. You are not interested in or couldn't be attracted by the opposite sex. You usually don't pay much attention on marriage and may be sexless. Instead, you like to focus more on your hobbies or career. If you are younger than 20, you may don't have a marriage line. As you grow older and contact more with opposite sex, the marriage line may appear. If you already married and don't have the line, the most case is that you don't have interest in your partner.
Good luck