I am sorry you hurt. I am concerned that in your hurt, you are not seeing clear.
You do not sound confused at all to me. You want to dial it back and assess and maybe even terminate the relationship with K. You are within your rights to do that if you want to keep agreements with your spouse.
K is making it hard for you to take a time out to think things out without static on the channel. That's called disrespectful.
Maybe also manipulative.
You had the ok for friendship from your DH. You had misgiving about K at the start because of his situation. You were sick and laid up and spent lots of time with him. It went to romanceship grey area. You need to discern. A time out. It's only been a MONTH here and it could be the brain dump hormones of a new crush clouding your vision.
It's hard to feel, but you know the ethical path to go. So GO have your time out so you can discern for yourself what you want and what it is your have here. Do you want to keep agreements with DH or not? Could align your own conduct toward that goal then.
If K makes it hard for you to have that time out without interference so you can meet your own need for a calm zone to discern in?
Could it be he does that because he want his
cookies from you and does not seem to care who he burns to get
them? Have you had time/space to consider that yet?
However "K" and I hit it off quite well and instantly bonded, which is odd for him.
How do you know it is "odd" for him? Because he says so? Could he just be saying that so you feel "special" and set up to be more receptive to his wooing?
The only thing is he's afraid to mention me to his girlfriend as she will get jealous and not understand the motive.
"My GF doesn't understand me." That old saw? Is that what this is?
WHAT motive does he have toward you that she would not understand? This one?
"K" on the other hand, knows for a fact that his girlfriend of 4 years will not understand, will be jealous, and will not accept him being involved in a poly relationship, and he's hesitant to even share that he has a friend in me, as then he'd have to explain how we met and why he was on Craig's List, etc.
Let's strip it of emotional text of the GF's... just zero in on his conduct.
"K" knows that his girlfriend will not accept him being involved in a poly relationship. He hangs around Craig's List anyway looking for outside partners. He is breaking /cheating on his agreements with the GF. He found me. Now he doesn't want to tell her about me because then he has to explain why he is breaking agreements and cheating on her."
There's the behavior.
Does he offer himself as a fine, upstanding, forthright, loving, and ethical dating partner or friend for you? No.
Does he offer himself to you as skulking around shenanigans dating partner/"friend"? Yes.
He also makes it hard for you to take a break or leave when you have determined you have strayed from your own "friend zone" agreement with your DH. If he "loves you" why does he make it hard for you to honor your agreements with your husband? How is this loving and respectful to you or his potential metamour, your DH? It isn't.
If he "loves" his girlfriend why does he break agreements with her rather than renegotiate or break up honestly because it is not a match?
So he does NOT do behavior that demonstrates respect for GF, you, or DH so far in his conduct.
He has however stated that if this is going to cause me too much of an emotional heartache, he understands if I want to back out.
Great! Could take him up on it! Because him saying that to you? That is STILL not saying THIS to you:
"We are facing some hard limits here. We must take responsibility for our behavior. We cannot go on like this and still be ethical. I love you and will not allow us to do unethical things. We must stop and sort our other relationships FIRST. Then see where this can grow. Let us not tarnish what we could share and could grow toward by doing unethical things at the starting line.
You cannot trust in me if you see me cheating on my agreements with my GF and not dealing with them honestly. I cannot trust you if you run around cheating on your agreements with your DH and not dealing with them honestly."
That could be saying to you...
"I do not want to be responsible for honoring my GF limits, or being honest with her. I also do not want to claim you as an open friend or open dating partner. I want to keep you in the shadows and expect you to help me to keep on lying to my GF even if it causes you discomfort or endangers your own agreements with DH.
I also want to shift emotional responsibility for our cheating affair on to you. I told you that 'if this is too hard for you emotionally, I understand if you back out.' If you stay, I'm off the hook for being held accountable to you or tending to your discomfort if this blows up and there's emotional fall out. Because I can tell myself YOU chose it. I'm a "nice guy," because I "gave you a choice." Rather than behave myself in ethical ways to begin with.
I just want my cookies. And I know I can get them out of you by pushing your soft gooey feelings for me. But I'll do it in a way that sounds good because if I just say "screw them, gimme cookies!" outright you could wise up and I get no cookies."
You noticed that, right? His weak ethics? His poor conduct? Not his sweet talking words. HIS ACTIONS.
K does not seem like an emotionally safe dude for you. Be careful.
At best, he seems wishy washy in his ethics and cannot be trusted to keep a limit / agreement in relationshipping.
At worst, he's a predator.
Neither sounds like good dating person or good friend material to me.