OK, time to get some wisdom from the "old guy".
This advise is geared to the fact that you are 18. If you were younger it would be completely different.
There are certain points about a relationship that can be a little grey, but there are some things that you need to have compatability. Money is one. You need to marry someone with similar finicial goals. Money is a big reason why married couples split up. And of coarse the next big one is sex.
Now the key is sexual compatibility. What I mean by this is that you need to know a lot about your partners sexually, and I don't mean that you have to actually have sex, but you need to know what thier sex drive is and communicate openly about it. If one partner is completely unsatisfied, a marriage/relationship will not last. You should be with someone with similar sex drive/desire as yourself. If you are compatible in this area, it will make your marriage/relationship so much better (trust me). This is not to say that she should do everything you want, it is to say that if she doesn't want to do anything at all, you may not be compatible. I would applaud anyone who wants to wait until they are married (sex usually complicates relationships and no relationship based on sex will last) but there are plenty of ways to stay sexually satisfied without sex (is she up for that?).
So what it comes down to is this. If you love her and think that it will be no problem to wait 3 more years for sex, I say go for it. But if your drive is too great and you think that either the wait will be torture or that you are having to give up something important to you, you should really think about who you are with. Complete lack of sexual activity WILL lead to someone cheating, because we are sexual creatures and the only unnatual sex is not to have it (when you are safe and mature, of coarse). This is not to say that you should pressure a partner into something. You do have to take into account that your labido is naturally higher as an 18 yo male. But if hers is an absolute 0, you've go some thinking to do.
Now having said all of this, I will share a little of my own history that may help you decision. When I was in college, I was a little irresponsible and let my smaller head do too much thinking for me. Anyway, it allowed me to get hooked up with a girl (an EXTREMELY hot girl, way out of my league) who's labido was stronger than mine. I thought (at the time) that sex couldn't get any better. I was wrong. I later met (and fell in love with) my current wife. We are very compatible in the bed room and the love factor makes the sex that much better.