I think he's asking for the Uke to fight for the relationship since the damn kid just told his boyfriend to go date someone else.
We say 'fight for a relationship' not to affirm to strangers you are dating or the relationship is strong but to affirm to each other that you love each other and that this is a trying time and we are going to stay in this relationship together.
But the Uke doesn't do that. He says, 'go date your best friend.'
Did you even read what the person say?
"but this is after he's been threatened by the seme's friends, had his lover blush and not flat-out refuse a confession, and had it confirmed by others that the seme is a better match with someone else."
You really think the seme and his friends are faultless? Please can you even see the circumstance or do you only support the seme? It's really ridiculous how you accuse the uke without even thinking about what he has dealt with. Are you always so narrow minded that you choose to focus on just the uke's actions than the circumstance behind it? I urge you to read again and analyse it again because your reasoning skills need work.
The problem with supporting the uke though is that he has absolutely no faith in their relationship. Satoi blushed because he was surprised by how smooth and persistent Mifune was with girls. People are always prone to chatter about how attractive looking people are good with other attractive looking people.
What's the circumstance behind the uke's actions? That he's paranoid of everything in his life ever? That he can't even think of trusting his significant other with the issues and doubts he harbors? Why is he dating the seme in the first place then? Relationships that focus on one person constantly having to validate the other person and have to reassure them they won't leave doesn't sound like a very good relationship. It sounds tiring.
Yes, Satoi and his friends, who by the way, are conspiring about everything behind his back and have not even tried to talk to Satoi about Soutarou 'two-timing him'- or the fact that they KNOW that Satoi is dating Soutarou, are being problematic. Subaru is planting so many seeds of doubt inside the uke's head. Mifune, with the whole confession scene, made the uke feel like he wasn't worthy of anything.
The uke shouldn't get to one-sidedly declare that their relationship is very weak just because he's heard some outside thoughts. Proper communication would do wonders for these two. It's a shame they don't know how to communicate.
I have this idealogy that you shouldn't have to "fight for" love. If two people are in love with each other, they shouldn't have to reaffirm their love every time one of them gets distracted by someone else. It's basically saying that unless you prove you love me more than they do, we shouldn't be together. And the seme didn't exactly help with how he reacted, blushing and saying nothing about being in love with someone else. Why does he wait until he walks away to revert to his usual attitude? Of course the uke isn't entirely free of fault, as he is weak willed and lacks confidence, but this is after he's been threatened by the seme's friends, had his lover blush and not flat-out refuse a confession, and had it confirmed by others that the seme is a better match with someone else. It's a cruel person that sees someone sobbing beneath them, asking them to not be angry, and accuses that person of being inconsiderate.