Or why I don’t think Damon and Elena having sex in 4x09 can be construed as anything but completely consensual. 

Basically, it boils down to that one time that Tyler bit Caroline because of the hybrid sire bond.

Until Tyler knew that he’d been sired, he acted as a mindless automaton and followed Klaus’s orders. The minute he understood and accepted the fact that he’d been sired – basically the minute he bit Caroline after he thought he’d successfully denied a direct order from Klaus – he ceased to be mindless. He started paying attention to his feelings, even when he couldn’t control his actions.

That, I think, is the key. As soon as Tyler was aware, he was able to know his own mind.

Did he still do as ordered? Yes. Did he still want to make Klaus happy? Yes. Was he happy or okay with either of those two things?

No.

Tyler was able to know his true feelings, even as the sire bond compelled him to act otherwise. He knew that Klaus was evil, even if a part of him wanted to please him. He knew that he should fight the orders even if he was essentially helpless to do so. He was able to voice his opposition to Klaus even if he ended up following orders in the end.

Once he found out about the sire bond, Tyler regained his free will, if not his ability to act on it. That is a subtle distinction, but an important one, imo, especially in terms of how I view the sire bond between Damon and Elena.

Speaking of which…

The sire bond between Damon and Elena is completely different than the one between Tyler and Klaus, even if it’s the same in theory. Accept this and move on. Seriously. Stop listening to antis and idiots and accept what the show is telling you.

Vampire sire bonds are different than hybrid ones. The end. 

They are also, as others have said, unique to the couple involved because as 4x08 told us, there have to be human feelings involved in order for a sire bond to take effect – I’d say the uniqueness is pretty clearly illustrated in 4x08 when you look at Damon/Charlotte vs Damon/Elena.

Now, the phrase dubious consent has been tossed around a lot lately, specifically in terms of Damon and Elena having sex again now that they know about the sire bond. Some argue that because the bond affects her actions that Elena can’t give true consent.

I disagree.

Now that she knows about the sire bond, Elena – like Tyler – will be more aware of her true feelings and she’ll be able to voice them. As Tyler said, the bond affects actions, not feelings. If things heat up between her and Damon – and from the looks of the promos for 4x09 it certainly looks like they do – Elena will have the ability to say yes or no to sex just like she would have before.

Elena still knows her own mind.

Her body may not respond, however. Depending on Damon’s words, Elena’s mind and body could be at odds with each other, but she still has the ability to speak, to give consent. If Damon wants sex and she’s just not in the mood, she can still tell him as much even if she’s simultaneously trying to shove her tongue down his throat. Damon knows about the sire bond – he’s probably going to be more sensitive to it than Elena is – and if he hears ‘no’ while her body is saying ‘yes’, it’s up to him to put a stop to it.

Quite frankly, considering his superior strength not only as a vampire but as a male, the situation’s no different than if both he and Elena were still human and there was no sire bond. 

No is no and Elena’s said that to Damon more than once since she turned and the sire bond came into effect.