Nature has made us to be in love for somewhere between 6 months and 2 years. Any relation lasting longer than that is a conscious decision - you choose to stay.
There is one thing you can't deny in a monogamous relationship... No matter how you try to deal with your desire to fuck others, there will always be hurt feelings and jealousy (if you actually love each other). That's why it takes a completely different set of skills to keep a relationship alive and interesting. Or actually it doesn't... Let me explain.
What you need to do to maintain a relationship is to stay fit, stay attractive, keep working towards your goals etc (be on your path). You also need to stimulate your gf emotionally, just like you do to in pickup to get laid. Both of you need to make an effort to make each other feel good together. It takes work, nothing comes for free, and it never ends. You cannot lean back and relax. If you do this right you will not only stay attractive to your girl, but to others too, and you will always have options and temptations.
So about marriage... Only marry a girl if you know for sure that she is right for you. You will only know that if you have lots of experience with other girls. It so happens I married my girl a few days ago, at Valentines day.
