The elephant in the room that has not been mentioned yet is that a significant number of Supercorp supporters (but not only them) have been constantly sending lots of very insulting social media messages (e.g. tweets) to actors (such as Jeremy, Chris, Melissa, Rahul), producers (including Andrew Kreisberg), other people involved with the show (like Sterling Gates, who was one the writers of episode 2x13 or Leanne Aguilera who has interviewed the cast or Kevin Smith who directed two episodes), and fans of the ship between Kara and Mon-El (Karamel) for months. These messages are not difficult to find in social media. In the week before SDCC Kevin Smith and Leanne Aguilera spoke about the insulting messages that they have got because of this on their Twitter accounts. Rahul spoke about the hateful messages that he received right after he was announced as playing Lena's ex in a recent interview. It has been and still is quite bad.
For the record, I am aware that a majority of Supercorp supporters and people who don't like Mon-El or his fictional relationship with Kara have not sent hate or insults to anyone. I am also aware of the reasons that people have for not liking the ship or the character. I do think that every storyline or character or ship in any show or movie or story can be criticised. But I also believe that any criticism can be done in a respectful manner. I have no problem with people having different opinions and expressing them publicly and trying to convince showrunners to change their storylines if they are not happy with them as far as they do not send insulting messages to real people. Some of the hate that has been sent has been extremely bad. I also think that people involved in the show and fans should be able to express positive opinions if they want to, that all ships, canon or fanon, are valid, and that everyone should be able to enjoy any show in the way that makes them happy.
I have also seen plans shared in social media by a big number of people (many of them Supercorp supporters) to boycott Chris' presence in SDCC and make him feel as uncomfortable as possible because he has spoken about the perspective of the character he plays, his character development to heroism and his relationship with Kara (btw Chris he got the loudest cheer during the Supergirl panel). I have seen messages shared by Supercorp supporters 'joking' about murdering Chris during SDCC so Mon-El wouldn't return back.
I wouldn't be surprised if the cast actions and reactions during that song, who were childish, had something to do with months of all this hate being sent to them. It is not nice receiving hateful messages so I don't think that everyone in the cast have happy thoughts about the Supercorp fandom and maybe they had too much fun with this particular joke.
I do think that the cast members who participated in that song shouldn't have done it, because the way they said that Kara and Lena were only friends and a romantic relationship between them will not happen in the show could have be interpreted as mocking because of the song and their reactions. I also think that this has been blown out of proportion because some of the Supercorp supporters were already unhappy about characters and storylines and statements from actors/ producers about the story that they want to tell. Maybe it wouldn't have been interpreted as mocking Supercorp and LGTB+ fans if the most vocal Supercorp supporters weren't already hating vehemently the direction that the show seems to be taking (Mon-El returning + Lena and Kara as friends and not a romantic couple).
I also think that some of the opinions and summaries that have been posted online about this situation as a response to people asking what had happened during that interview are not true or biased. For example, Chris was the one cast member in that room who didn't laugh or react during that moment (when Jeremy said that they were just friends) but a lot of people is saying that Chris was mocking fans and Katie was not. Full disclaimer, as a Karamel shipper and Chris Wood fan I am probably biased too. So the best way to get an opinion is watch the video.
I have read many social media messages from LGTB+ people on this issue trying to really understand their perspective. Many said that they think that this wouldn't have happened if this wasn't a wlw ship so there is homophobia involved. Many said that this has been really bad for them because shipping LGTB+ ships can be important for people struggling with their identity to help them accept themselves and feel that they deserve happiness and love. Many said that shipping is sometimes the way some LGTB+ people use to cope in difficult times, and they are crushed because Supercorp was their safe happy place and now it does not feel safe and happy anymore. Many said that they are feeling terribly bad because they have interpreted this as a non LGTB+ person telling them that the way they are and their feelings are not important or valid. Many said that because there is discrimination against LGTB+ everyone should be specially sensitive and not saying things that could be affecting negatively LGTB+ people. Many said that this situation has been a trigger to them reminding them of bad experiences. Many said that non LGTB+ shouldn't be the ones to decide what is discrimination and how to feel about the idea of two women loving each other. I am listening to all these opinions and think that they have good points.
So this situation has been a mess and it would have been better if that joke during the song hadn't happened. And I think that apologies were necessary because feelings were hurt.
In their defense, the cast spent all day joking about everything in all their interviews. That same day they also joked about Karamel and made fun of each other. I don't think that what happened at that interview was planned, it was just an improv song and it happened.
I also think that Jeremy is a good guy and he feels truly sorry. He has listened to the feedback that he has received and I don't think that he would do something similar in the future. I think that it is difficult to understand what is being part of a minority when you are not part of it and he has probably learned from this incident.