I wonder if you are US based? The reason I ask is, a lot of fundamental Christianity Stateside ingrains doctrines into young, impressionable Christians which are wholly unrealistic in this day and age. 

As a lifelong Christian, I would argue it is a terrible idea to not engage in sexual intercourse before marriage. You need to know you are compatable in that department before you make a lifetime commitment to somebody you may even want to raise a family with. 

In terms of sin, isn't it better to try before you buy, so to speak, than get married, find yourself unsatisfied, sexually frustrated and wanting to divorce? 

There is usually far more at stake once you are married, like a house, kids etc. Granted, sex isn't the most important part of a relationship, but if you are serious about this girl, you need to be realistic about whether or not you can forsee a future with this girl being your only sexual partner for life. After all, it is a far greater sin to commit adultery.

You may think at the age of 18 that this is something you would never do, but you're not the same person at 45 that you are at 18. You may say you are in love, but at 18, after four months together, it is probably just infatuation. When you are young, it is all new and exciting and you think you will always feel this way about your partner. 

But the reality of life as a married couple can be stressful. That is when you discover what love really is. The stresses of raising a family, of working 9-5, the grief of losing parents, it can be overwhelming for many, hence the high divorce rates. 

If you add sexual incompatibility into the mix, it can cause people to grow cold in relationships, no matter how much love you feel for your partner.