Responses
This is a very good explanation, somehow prediction. I like how keen you are in the details swirling with the two and, in all honesty, I was nodding in agreement with all that you said since I think the same way. I am hoping that both of them will be able to settle their differences with a heart-to-heart talk. Nice one! d(^_^)b

I think maybe Keiichi and Shingo have different expectations for what a relationship should look like long term. Putting aside Shingo's abandonment issues for a moment, I think Keiichi doesn't realize he owes Shingo an explanation about Angelo, and that it never occurs to Keiichi to talk things over with Shingo before deciding to leave the country. It seems like a Keiichi expects a rather old fashioned relationship with a power imbalance where one person makes all the decisions and takes for granted that the other will trust and love so much that there is no need to talk about it and make decisions together.
Now, I think Shingo should correct this impression and not jump straight to breaking up, but at the same time, you can't expect a modern person who wants an equal relationship to accept "I love you" as a reason to be treated like a second class citizen. For Shingo, the problem is abandonment issues--and Shingo has to learn how to cope better with those. But for Keiichi, the issue is making decisions on his own and expecting Shingo to passively go along with whatever Keiichi does no questions asked.
If Keiichi was planning on proposing this entire time, why not talk with Shingo like an equal partner? What kind of marriage was Keiichi thinking about when he made the decision to leave the country and didn't even talk to Shingo about it before hand? I believe Keiichi really did want Shingo to wait at home like a good little kitty, but I do think Keiichi, of all people, should know the difference between cats and dogs. You want unquestioning devotion and unchallenged authority over the other, get a puppy.
But I am still willing to blame Angelo if we are given a good reason.