Because it's the general chat forum? Anyways, sometimes strangers give better advice than people who know you since they're the only ones who can give an unbiased opinion.
I disagree with the bit about stringing her along, if your girlfriend wants to marry you and you know it won't happen that's one thing, but that probably isn't the situation. From what it sounds like, you're both kids who have been together a long time and that's that.
As I said before, there's nothing wrong with having a little somethin' on the side. Americans are very possessive of each other in relationships and cheating is really frowned upon, but in most other countries in the world it's pretty much OK as long as the other person doesn't find out. Be careful that your girlfriend doesn't find out so that it doesn't ruin things for the both of you, but remember, what she doesn't know can't hurt her.
I've talked to older guys that really regret not getting to play the field when they were in their prime because they were tied up in serious relationships and felt like they couldn't cheat. If I were you I wouldn't make that mistake, life is short and you only live once, and you have plenty of time before you're married. TBH I would strongly recommend getting married in your mid 20's, especially if you just got out of college. Enjoy your youth as much as you can while you have it.
Sleeping around doesn't guarantee happiness. If you want to **** about then do it, don't sulk in a relationship at the same time though.
Oh and cheating on someone is horrid no matter what nation you are from. To say that it's accepted is moronic.
'See no evil, hear no evil' is a ****** coward's way through life.