Your friend is right. You are just 19 years old and this is not the time to think about marriage and serious relationships. I would say you’re lucky that this guy seems honest and gave you good advice otherwise you could have easily become a victim of some kind of exploitation or abuse. You have your whole life ahead of you and you will meet the right guy who will be suitable for a relationship. Concentrate on your studies and career for now and just treat your FB friend as a good friend.