1: Lets say you meet a nice girl, things go great, you talk about marriage, you're 4 years into the relationship and even living together, then suddenly she cheats and you leave her.
In those 4 years you could have found THE wife material that would have never left you and stayed with you forever. but you'll never know cause you spent all your energy into this girl.
2: same as # 1 but lets say she doesn't cheat and you guys end up married and happily ever after (serious, no fights or break ups ever)
well you're missing out on the HUNDREDS OF THOUSANDS of hot 8-10s out there that you could mess around with and have fun with (both sex and just normal fun) but you would never know cause you're tied down to one girl
I used to be an anti social guy but i've been going out more and my mind = blown at all of the potential awesome long term friends you could meet out there, just by stepping outside and talking to people at a big social event (concert, some sort of gaming con, etc etc)