You sound like me when I was bashing Murong Feng in the TLTSILY thread. Haha.
Yes, Kuk Wai did sleep with Ling, but only because she drugged him with aphrodisiac incense. In his confused mind, he probably thought Ling was Yan. You could say that Ling raped Kuk Wai, as he was too drugged and mentally altered to give consent.
As hinted in the scene between Kuk Wai and Ling, Ling became pregnant after that encounter and Kuk Wai HAD to marry her. But then, shortly thereafter, Ling lost the baby. Anyway, it really didn't matter whether Kuk Wai slept with Ling willingly or was forced. Yan still couldn't forgive Kuk Wai. Sometimes, there are certain things that could never be forgotten, no matter how hard a person tries to forget. Tweety, being a staunch Wallace Huo fan, feels that Yan still loves Kuk Wai, even though Yan could never forgive him. Tweety could be right, but this could all change, depending on how persuasive SCN is. Sometimes, it's easier to move on after a past hurt if there is another person waiting to welcome you into his arms.
The difference between Kuk Wai and Murong Feng was that Kuk Wai was drugged into sleeping with Ling, whereas Murong knowingly got himself inebriated with alcohol when he should have been by Jingwan's side, supporting her though her emotional turmoil after the miscarriage. Instead, he chose to escape through alcohol. You could argue that the alcohol made him sleep with Jinzhi, but I think that alcohol only loosens inhibitions. If the desire to cheat with another woman isn't there in the first place, he still wouldn't have slept with Jinzhi no matter how loosened his inhibitions are. And even if we are magnanimous enough to forgive him for that one indiscretion, then how can he explain his sleeping with Jinzhi a second
time, which got her pregnant? Sometimes, I feel that Murong Feng is even more despicable and unforgivable by sleeping with Jinzhi, getting her pregnant, and then still treating her with such neglect and meanness. He betrays and cheats on Jingwan, and then when he meets up with Jingwan again years later, he wants to drop Jinzhi (who is pregnant with his child) without a thought! This guy is truly a monster.