For all the left people I know, including myself, The reason we don't want a line drawn is because sometimes special circumstances arise. There may be medical complications in the third trimester that would result in the mother's death and it's not feasible to exhaustively list every scenario that could land her in this situation so it's better to just not a put a limit on it so she doesn't have some bullshit hoop to jump through later while she's dying.
That said, I don't think there's anyone genuinely arguing that people should be allowed to get abortions super late into the pregnancy just for funsies. Third trimester is the logical cut off to me, and most of the people I know agree or want it slightly shorter. We just don't want the law to specify that since it can cause legal complications. It's better that it be considered a medical standard.
I'd like to add to their comment. A lot of people are coming from very argumentative or abrasive communities, and sometimes their initial messaging will show that. If someone comes off way too hot headed try not to assume that they're going to end up being a troll. They might just be riled up. I've had pretty good success in asking clarifying questions to get people to explain their view rather than just come in swinging.