Some of it is mostly me reading into things and frowning, but Bhaal being able to violate Durge sexually and the intent of sexual exploitation (regardless of Durge's will) is canon and it does lend credence to the idea.

Durge is obligated to breed, and Bhaal can and will force them to; the original version of the feral ending in the dialogue files involves him forcing them into a state of sexual hyperarousal and forcing them to mate with a gnoll, in the same way he can force the urge to slay on them. Bhaal's intention there is to see if he can breed more monstrous Bhaalspawn. Some of the dialogue options are just Durge screaming in horror.

Durge has no memory of the actual rape; Bhaal forces them to spend the entire time thinking about him: "Your memory of last night's act is absent. In the moment of mounting, your mind emptied itself, and you could think only of Bhaal."

And knowing that, when I saw "I love you father. I'm a good [child], a good, good [child]" in the released version I had to go sit down for a bit.

We know that Durge is expected to have kids - it's a mortal sin that they haven't. It's also vague whether they tried or not pre-amnesia; they're described as "failing" not refusing, it could've gone either way. Durge may already have been exploited. Even if they don't disappoint Bhaal in game, this "duty" remains. It's not just the Tribunal making shit up; Sceleritas is your direct liaison to Bhaal, and he makes this clear to them as Bhaal's Chosen too.

We know that if Durge won't kill then Bhaal will force them to, and we know that he can force them to have sex too. It's entirely possible that Durge's only say in this is to either fuck partners they chose or have them chosen for them (as we all know, Bhaal has zero respect for his kids' bodily autonomy or desires past his use for them, and Bhaal wants more babies.)

At the very least, it doesn't seem to be a major part of Bhaal's plan for Durge yet, so Durge may have dodged this specific form of abuse for now.

Bhaal has also done this to Sarevok, it seems, who would probably have killed himself back in BG1+2 if he could see what would happen to him in his future.

Knowing that Bhaal has no issues inflicting sexual arousal on his own kids, those scriptures you find that explicitly refer to the pleasure he bestows on his followers when they kill as "erotic" unfortunately have little reason not to be applied to Bhaalspawn. It would also explain the necrophilia if Durge has been conditioned to see killing/corpses as sexual.

Not quite the same thing, but the thing where people think Durge fucked the Netherbrain is actually from a remark by Bhaal in the journal update, where he makes a crack that "That slimy thing in the Moonrise called you its fallen star. It was very reverent [...] apparently [you're] good at playing with tentacles."