To answer you're question, Yes! Yes you are an illogical idiot! Putting aside your dick's desires for a moment, take a fucking look around at the shitstorm you're in. You already know the answers to all these questions. That's not why your struggling so much with this decision. Your issue is that you want to have your cake and eat it too. You're scared because it might not work out for you if you make an actual decision. There's no contingency plan in the world that would enable your GF to trust you again after this.
You want someone to validate your feelings and tell you that it's okay to want other girls and your GF should allow you the space you want to fuck this other girl, maybe dabble in a relationship to see what she's like, before you make your final decision with the approval of everyone involved and they all never speak ill of you and think your the great guy they have always known. Your friends who are encouraging you to cheat on your GF did get one thing right; Someone's heart will get broken in all of this and the blue Jay's of the world are going to be singing at the top of their lungs because you aren't even going to be able to breathe when you realize what you did to someone who gave you five years of her life along with her heart. Once the realization of what you have done hits you, your going to be a shell of the person you once were. It's a damn shame too, because even though you're trying to warp all these subconscious feelings to your what your dick wants the most, I get the sense you were probably a nice guy once upon a time.
Which brings me to why this decision is hard for you. It's not because you are afraid of hurting your GF, but because you want someone else to make the decision for you. You are struggling with this because you don't want to be the bad guy.
Well grow up and stop agonizing over your terribly long and difficult decision because not once in that entire monologue did you even consider how this is going to affect your GF as a human being. You don't want to fuck up her career, sure, but what about her life. What about her ability to trust. What about her ability to feel like she's enough for someone. Pretend sexual tension girl doesn't exist for a minute (if that's possible). Take a moment to really think about the girl you spent five years with. If you think you gained everything you are going to gain from this relationship, then end it. This girl deserves at least the dignity of you telling her the truth about why you are ending it to. You owe her that much.
You want an open relationship, but you don't even know whether you want meaningless sex or a relationship with this 28F.
Open relationships are for people who aren't afraid to be honest, which you can't even be with yourself right now. Not to mention all of her friends are going to hate you when they find out about this and when they finally do bring her back down to reality, she's going to hate you too.
You mentioned your on again, off again relationship and I get that this is something you did in the early years of your relationship, but you are 26 years old. Your girlfriend is 25. If she wants to get married one day, you need to let her go find someone who WANTS to commit to her, because that is love. You wanting to "Test the waters with 28F" while keeping your relationship open, all so you can be sure you make a rational decision about whether or not you want to commit to your GF.
You might as well just slap her in the face. I don't like telling people how they feel, but I can tell you right now, you don't want to commit to her. Because if you did, you wouldn't be looking at any other girl, let alone inviting the ones you are sexually attracted to into your friendship circle, ensuring more one on one time for the tension to grow and excitement to spark. You just don't want to end up alone, so your GF is your safety, which sucks, even though you consider yourself a catch. Maybe you are a catch, but if you so much as touch that other girls arm in a flirty way before you allow your girlfriend to have a tiny crumb of your respect, you become a douche bag extraordinaire.
Also, you need to own up to your shit. I was depressed when I moved to a new city, yet somehow I wasn't trying to create a ridiculous sexual loophole that lets me cheat on my boyfriend. Blame it on depression, moving to a new city, whatever you want, but at the end of the day, YOU don't want to commit to someone who loves you because you want to see what else is out there. You literally said, and I quote:
"I think 28F might be interested in something more. I have rationalized this as myself believing that I am a "catch," but I am still unsure if she actually feels this way."
So basically, if she made her feelings about you clear and expressed interest, you would drop your GF in a heartbeat to start a RELATIONSHIP with this girl.
I think meaningless/healthy sex would alleviate the stressful amount of sexual tension we have and hopefully might allow me to make a rational judgment of staying or leaving my relationship with 25F."
This is not meaningless sex. You are describing the beginning of a relationship.
Look, I'm sure your a decent person. The point of this entire post is to show you how self-centered you are being and prevent you from becoming the largest cum wad anyone has ever laid their eyes on. Have some respect for the girlfriend you have spent five years of your life loving. You don't need to make this dramatic.
Do you love her? Are you willing to give her up forever? Can you imagine your life without her?
Relationships aren't friendships. You can't have a little of everything and decide who your best friend is later. You said you have a problem committing to her, but you only gave excuses. You can have her, or you can't have her. Sacrifice and compromise are so misunderstood. You think you're giving up something, you're not able to do something, but you're getting something that very few people experience in their entire lifetime. Love is not something you should fuck around with. Be honest with her and tell her the truth about what's going on so she can decide for herself if she even wants to be in this anymore. Imagine if she were doing this to you. And for the love of all that is holy I will hunt you down myself if you cheat on her.