There's nothing wrong with presenting your problem in a jokey tone. It's a perfectly natural defence mechanism to make a bit of fun of yourself. I still take you very seriously because you've come and asked for help, which you wouldn't do if you didn't need it. I think you sound like a great guy first of all. You seem to have a good sense of morals and you're fairly smart. You know that you don't want to interfere with someone else's relationship situation and you don't want to screw around with people's feelings. These are all great things and you should give yourself a pat on the back for them. Well done. Lots of guys never get that far so you have one in the bag already.
As for these girls....
I don't think you should feel too guilty about messing around with the second girl because like you say you've been kind of distant from the first girl for a week or so after she told you that she had kind of messed around a bit. I think you reacted pretty reasonably and you were confused so sure you might want to feel a little bad that you could've done things a bit better but you weren't solely to blame on that one and things were confusing.
The first girl - has she spoken to you again about what she wants from you? In fact that's my advice to you woth both these girls to be honest: speak to them. Ask them what their intentions are. You sound like the reason you're confused is because you're trying to make a decision based on a pile of assumptions and guesses and that would confuse just about anyone if you ask me. You seriously need to talk to both of them and find out what their intentions are. Is girl 2 ready for another relationship yet, or was this rebound or just a fling because she was enjoying being away? Is girl number one actually interested in being exclusive iwth you or does she still have feelings for this other guy? I don't think you can decide which one of them you want to comit to until you actually find out how they feel and what they both want, and I don't think you've done that.
If you like your relationships to be based on friendship first then maybe that's what you need to do here - tell them both to slow down because you want to get to know them. Then maybe you'll find your decision is made for you because you'll find you don't actually get on that well with one of them. If either one of them isn't willing to respect your need for friendship first then again maybe you need to consider if she is really someone you want to be involved with - after all it's not much to ask of her!
But yeah, basically I think you're trying to decide prematurely. I don't think you have really found out what all your options are or what all the possibilities are. I don't think you CAN make this decision until you clear a few things up, and I think the only way to do that is to communicate. Otherwise you're going to stay confused.
If you're worried about HOW to talk to them then I'd say just go out for a coffee or something and say you want to talk about a couple of things. Explain you're kind of confused about what happened and say that you really want to respect their feelings so it's important that you know what those feelings are. Tell them you're interested in them and want to know hat's going on inside their head. They'll respect you for that and they'll probably give you some surprising honesty.
Good luck, PM me if you want to talk more.