Question, KeeblerElf: Do you live in the Southern hemisphere? If not, don’t worry about a possible summer relationship with Susan. In eight months or so, everything will be changed.
If you do live where summer is almost upon you, do what Giraffe said. I mean, what’s a few good friends or a steady girl compared to some summer finger-action? Let’s get our priorities straight here! 
Sarcastic tone not aimed at you, Giraffe. Your advice is spot-on for the circumstances as described by the OP.
Now how many people want to bet that he doesn’t follow any of this good advice and do whatever it was he was thinking of doing before he even posted here?
(btw I’m someone that’s bitter about people constantly coming to me for advice then having them do exactly what they were going to anyway)
Darkhold, I’d probably bet on that. If the OP follows the advice of his little head rather than that of his big head (plus most of the posters here), he’ll probably follow his basest instincts.
Happens all the time, right?
So Chandler, you’re happy at the moment with Monica and you see no future for Rachel and Ross (well they were drunk when they got together) and Joey has made a play for Rachel and you want to get in her pants too. Wait 'til next season.
He said as much:
Twenty something posts later and he hasn’t rejoined us. I would say he isn’t that interested in anyone’s advice. But if you are, KeeblerElf, I’m going to add to the general consensus. RUN because it’s just too messy for you to escape with no regrets.
Keebler, from this moment on you begin to develop the fiber of the moral character which will be yours for the rest of your life. Another way to think of it is that you put yourself together.
Your respect for yourself is even more important than your friendship with Susan. What choices will lead to your own self-respect?
Much of this will take care of itself before next summer. Step carefully so as not to hurt yourself or others.
Enjoy that sweet and strange sexual tension between you and Susan. It is life’s finest wine.
Best response I’ve heard yet…
[sub]//Blushes, lowers head, hands awkwardly fumbling about in pockets, kicks pebble in general direction.[/sub]
Aww, shucks, you guys and/or girls! Thanks! [sub]Ooh, what’s that warm tingly feeling?![/sub]
KeeblerElf, I hope you won’t overlook the general message here. You’re not thinking with your head. People will get hurt. You could seriously destroy the relationships with those involved. If these people mean more to you than a fling, you might want to go into damage control mode.
From John D. Macdonalde’s Travis Magee series…
Meyer’s Law, rephrased from memory: In every situation, the choice that is the hardest one to do is the right one.
Honestly, you of high school age do the silliest things. You aren’t quite sure what you want as the evidence with Susan suggests. She will be physical with you and she is willing to date(?) this other guy. I sense a hint of love, you for this Susan, and so this situation will hurt you in the long run.
Does she know that you… know about that conversation? Be honest and ask that she be honest with you. If you want something more with her, this news just does not inspire my confidence that you’ll attain it with her. Don’t expect much, is my advice to you.
My advice
When you make up names for people in your story use a different letter of the alphabet for each person.
I usually follow the ABCs. I think this makes it easier to keep the characters straight.
As for the situation.
Look at yourself in the mirror. Decide if you are the type of person who wants ‘closed’ relations or ‘open’ ones. If ‘open’ then everyone involved in the relationships need to know about the others and be ok with ‘open’ relationships. (that means Shane and your gf must know about it)
You might think that an open relationship is morally wrong but that would be your moral choice. People do this sort of thing all the time.
As attractive as this girl must seem to you (especially being “forbidden fruit”), everyone else already told you what to do, run. Well not really run, but distance yourself. My bet is that the couple breaks up soon enough and you can move in without causing too much more drama if you can be patient.
Enormously entertaining responses at first, excellent advice towards the end. Thank you very much. Maybe I’ll end the story some day…with possible threesome details…