i might missed it but why couldnt datho reject will's advance again?

I think it’s bc Will didn’t have any real intentions behind it. He was just joking around. Like when Julio kissed Daath on the cheek, it was friendly.

It's not that he couldn't, the problem was his body didn't react to it. Usually when someone came onto him his body would strongly reject the other by reflex to the point of vomiting since he was already bonded. But that wasn't the case with Will, which confused him. But it was because Will had no sexual intention while doing that, so his body didn't reject him.