
He already explained why he said this would be a one-night stand:
1. he wants to make up for the rough "use" of him previously (ie, he really misunderstood our uke). So, he wants to let him have a good sexual experience to blot out the bad one, sorta.
2. it's not proper to have relations with a subordinate in the company (ie business ethics, I guess).
Now, we know they will be a couple, duh hahaha, but he's laid out his own feelings. He has guilt about roughly using our cutie, and he has business integrity and it looks bad to have an affair with someone under you in the company.

Oh, I know it won't be a one-night stand. And we already see him having admiration/like/desire for our cutie. But it's one thing to THINK a thing is the right behavior, and another what reality brings about. Kang has been straight all his life, just recently broke up with a lover. He found sex with our guy pretty good even when he looked down on him. Now, he's determined to be extra good, extra thoughtful as a lover to make up for his previous shit behavior. Hence, it's to be expected the sex will be respectful (not rough or mean) and tender (apologetic) and passionate (let's face it, he got a boner even when he was disgusted by his opinion of uke, now he has a great opinion and is gonna boner even more.)
This is a love story, and Kang is a bed beast. But he also has a pretty good heart. :)

See I had a theory that maybe he’s not comfortable or familiar with the fact that he’s about to bang a guy (plus the whole being his boss thing) so he may just be resorting to like a business exchange approach he may just be trying to cut it off after that so his feelings don’t progress further and it doesn’t effect their lives more than it should (in his mind)....I think the whole “walk the line” thing was to reassure him that Yiyoung wants to fuck and actually means it so he doesn’t do something job risky if he was just wasted

I guess I see him as pretty honest and with a strong ethic. He said clearly he was TEMPTED by the offer of equal sex. He made sure cutiepie was not so drunk he was not able to properly consent. He even clarified the terms (similar to the ones Yiyoung himself uttered--one more night as equals, not as bargain.) Kang is actually responding to what Yiyoung wanted in the car, verbally and by grabbing his crotch. But he's doing it like a mature man--getting consent clearly, setting up parameters (the rough sex and kiss thing), clarifying expectations, and explaining why it was "one" only: he was not in the habit of having flings with coworkers. Considering he's a biz partner and BOSS here, it is problematic to a person with a particular clear-cut idea of what one should and shouldn't do with coworkers. He apologized for misjudging Yiyoung, and now wants things CLEAR....
That he obviously is falling for our guy we see, and he'll have to face that. For now, I don't think he's being uncomfortable about anything. I don't see him hung up about banging a guy when he's only had sex with women. He said it at the hotel, how it was surprisingly pleasant. He never showed angst or shame about the sex. His shame was about mistreating/misjudging Yiyoung.
He's a very cool, up front, manly, sexy character, who is about to fall head over shiny heels for our delightful, sweet uke.
The seme wins the award for the most civilised, mature behaviour...he didn't abuse /drunk rape the uke he refused the suggestion like a mature man...he didn't give false promises to uke he settled rules and boundaries before initiating anything(although I don't understand him suggesting a one-night-only since I'm sure he is going to ask uke again)