Disclaimer: I haven't read the others replies. Just the original post.
My advice? BREATHE, BREATHE. Remember nobody is going anywhere yet that is past the point of no return.
Emotion is just internal weather. Rain is rain. Sun is sun. Emotion is emotion. Some of it is yummy to feel. Some of it is yucky. Let it blow on through.
So you found an area where it feel yucky. Imagining him with another lover feel vomitous to you today. Ok. Acknowledged. Plant a mental flag there.
You could call this leg the "just ID things" part in your mental journey as you contemplate pursuing this friend. You could give yourself permission to just BE at that stage right now. ID and flag for deeper assessment later. That could help reduce the "overwhelm factor." Expect LESS of yourself. Be OK being a newbie here.
Maybe this one area always will bug you. Maybe it will feel different tomorrow, next year, next decade. That we cannot know for sure at THIS point in time, right? So just... ID and flag.
When the time comes to assess and contemplate possible solutions... You don't have to DO anything about emotions if you do not want to. That's choosing to let it just... pass on through and give it time. Sometimes the passing of time solves it, or changes it so it's not so hard to deal with.
You could choose other behaviors to address your emotional management too -- talk to a friend, write in a journal, not pick at it like a thought scab, etc. It's good you try to write in this forum. WTG!
Maybe you want a blog thread so collect all this in? In the blog thread area?
Try different behaviors to see what works, then remember it for your "emotional coping toolbox." Stuff that doesn't work -- well, stop that and try a new behavior.
Am I being utterly unfair in this? Does this make me a terrible person?
No. You are experiencing lots of new thoughts/feelings, and some of it feels yummy (like the crushy feelings for your friend) and some of it feels yucky (like dealing with the thought of DH with another person.) That's just Life. Some things are fun to feel. Some are not. In the mental exploration of all this new stuff -- I think it could be easier for you and your anxiety level if you could acknowledge that to yourself. Something like...
"Yeah. Some of it feels fun and some of it just doesn't. But this is all in my head, a safe space to try things on. NOBODY is being hurt here. NOBODY has done anything here."
If it helps you feel any better, in my own Opening Talks with DH I told him point blank "Well, if we're Identifying Options and Possibilities? The easiest one for ME is being the hinge and no metas on the V arms. Tada!" and he just laughed and agreed. Just 3 people in the polyship to have to accommodate. More people in the mix = more wants, needs, and limits to have to coordinate and try to meet. That doesn't make me a horrible person to state it and simply acknowledge that it's the easier workload on ME. It would be!
I've pretty much pinpointed my UGH with imagining DH with another as fear of the unknown nutjob stranger. He's precious to me, and it makes me feel grody to think about him being hurt somehow. I imagine with people I already know, or close friends and there is no vomitous. I imagine him with someone I don't know and here it comes -- Anxiety Train. So I don't know if sharing that helps you as you sort out your feelings.
I have anxiety myself -- so for anxiety management? Watching out for emotional flooding times? Or panic attacks? Some uncomfortable thoughts that I have, I just have to vulture. Think it, let it go, think of something else. Think it next time for a bit longer, then let it go. Like spiraling around it and coming closer for a stab at it and them fly off. Because it's too big to take on straight up. Break it down into smaller bits. I don't want to trigger a THING -- either an emotional flood or a panic attack and then have to deal with the original thing and then THAT too!
I don't know if any of that helps with anxiety management.
But remember to BREATHE. You don't have to know everything all at once. And the not knowing? It doesn't have to be disaster. You are safe, with your safe people. You are not in danger. Could remind yourself of these things and do a little self reassure/TLC when you need it.
Hang in there.