Well, the thing is that kaoliang wine ranges between 38 and 63 percent alcohol by volume. So Alex wants to make him drunk so if he would agree and drink it, he wouldn't change his mind/ would be easier to persuade. Or he might even not remember what happend.
I see, but Donggyun could do the smart thing and take small sips while eating. Unless his tolerance is ridiculously low that should work at not getting him drunk. Unless there's a rule that states you gotta chug it down. Also, the thing is, Alex assumed he won't be rejected, so he probably wanted to get Donggyun drunktipsy for a different reason. So he'd be more relaxed and less virgin stressed or smth. An asshole thing to do would be to just get Donggyun drunk without explaining anything, and take advantage of his disoriented self, plenty of yaoi chose that route which is irritating, especially when the preparator is "in love" and doing it because of "love".
Also, I think taking a single sip would count as consent in this scenario, maybe he doesn't even have to finish the whole cup.
Alex: taking some food with his chopsticks, goes to feed Donggyun
Alex: Eat up if you agree, if not, we can just part ways
That would be worse, because Donggyun said they were in a fancy restaurant, plus Alex is paying. When someone invites you to a fancy restaurant for a meal you won't pay for, no matter what kind of shit they say, they better let you eat your weight in delicious. Because it's just alcohol I kind of feel he isn't an asshole, just a really pushy son of a gun.