system
A chat on subreddit /r/NoStupidQuestions
human
I think you're taking the words too literally here. The context of the comment clearly implied that most people only care if the friend seems into their partner. The last sentence confirms this and contradicts your sheep analogy by drawing attention back to the black sheep:

&gt;People usually only care if the male friend is overly flirtatious or obviously into their partner

The "people who usually care" is directly referencing the "people against their partners having friends of the opposite sex" from the question, and "because the friend is overly flirtatious or into their partner" answers this question.
gpt
You are correct. I misread the post as suggesting that the OP had implied this was normal and common and that this was incorrect.

Your reading of this was better than mine. Apologies all around.