..Did you not read that conversation between John, Dave and Karkat where Dave was visibly jealous by Karkat bringing up his past black crush on John? Or remember when Dave broke up with Terezi BECAUSE he was uncomfortable with her dating Gamzee at the same time? Also that part about penis ouija, right before that Karkat had a conversation where he said he wanted her "in every quadrant" specifically so that he "could have her all to himself" and compared it to joke characters in troll romcoms
A. I don’t think it’s that uncommon for a person to get jealous or flustered or whatever when their partner brings up an old flame or crush
B. Dave broke up with Terezi because she didn’t tell him she was also hatedating Gamzee and he didn’t understand it.
C.That doesn’t really factor into potential polyamory but rather highlights Karkat’s weird obsession and idealization of Terezi