Why stay faithful?

this is idea i debated with a friend, who is in a relationship but feels they don't have to be faithfull cause they don't know if that person is the one yet and feels that since thier not married yet it's no real ties.... so what does that mean.....That's the idea, let's say your in a relationship and your looking @ it like i'm with that person but were not married, which is like saying were together but not for long basically.... so what would be acceptable rules in a relationship while your with somebody, because if the one your with is notconsider a serious relationship, their is no need to look @ that relationship like their will be someting more than, what it is, it's basically just a steady fuc^k partner for that specfic time in your life...... so is it really needed to be faithful to that partner and give all of you to that person knowing that oneday ya'll time would come to a end, anyway. then your bac^k to square one and would go on to search till you find that person your ready to settle down with...