You're going to have to elaborate in what way you mean "incestuous grooming", considering both of those terms carry sexual connotations AND grooming of their own children is something gay men get accused of A LOT. In fact, the idea that gay men will pressure their children into being queer is something that is also used a lot, and as much as we can argue Davekat is intended, Dirk is literally there in the story, pressuring Dave to have sex with Karkat.

I don't think I have ever heard the term incestuous (especially involving literal family) to not mean sexual? Maybe I'm mistaken, but even Googling doesn't offer some alternative beyond using incestuous to mean close or proximal LIKE family.

I don't think "gay character does something bad" = homophobia. I think that specifically choosing the characters whose sexual orientation has become a focal point of their character arc to turn into homophobic tropes is homophobic.

And I'm sorry, but I think you're offering a lot of good faith to the same author who included a heaping load of slurs in the early comic and didn't really choose to explore one of the protagonists queer identity in any serious capacity until the literal final chapter of the original run.

Dirk's characterization has always existed in contrast of his abusive behavior as Bro. Dirk's identity as gay was a retroactive decision, I don't think Bro being gay was ever a consideration when Hussie first wrote him, but it was added to "subvert" the idea of the dudebro that they had written; without much consideration given to the implications of making a gay man like that.

At the time, I think a lot of people were willing to overlook the implications, because Dirk existed in contrast to what Bro, who was arguably not really fleshed out, was retroactively now representative of. However, given that the epilogues now make Dirk 1:1 with Bro, I don't think I can actually overlook it.

It feels homophobic and I would go as far as to say that it actually is.