You must be mature enough to accept obligations to yourself, your loved one, and your family. Sex adds even more responsibility to a relationship. An unplanned pregnancy and bad grades may not seem terrible right now, but could ruin the rest of your life. Sex alone cannot make a relationship work. Sex is not love.
Teenage marriages have an extremely high divorce rate.
It is unrealistic to think that marriage may solve any problems that you have with your parents, in a relationship, or in school. For teens, marriage can create more problems than it solves with the expenses of rent, utilities, food medical bills, insurance, taxes, and clothing. It holds enormous obligations. It is not selfish. It is fulfilling the needs of another and finding enjoyment through giving, not just taking.