The whole white husband losing his wife and getting cucked by BBC is it's own whole fetish on top of just the cheating. It's basically like a second tier for cuckold fetish.
The reason he puts up with it is because he's a cuck. At the beginning of the game, the choice isn't about if he has the fetish. The mc's fantasizes and then makes a choice of whether or not to share that fantasy with Julia. Cornell, as we can tell from his previous partners, likes conquering other men's wifes/girlfriends, so he's naturally going to try to take liberties because it's what gets him excited. Now he starts small with easily excused things like a kiss that's a bit over the top on passion, but as he does more and sees the mc continuing to give him the green light, the more uninhibited his actions become. Julia won't ruin it because she's enjoying it all, and the mc won't say anything because despite never seeming to have explored his fetish before, he's loving every new experience.
I have to say, I am quite happy with the cuck/sharing path being split into separate cuck or hotwife/sharing paths. Despite having played the other games, I don't remember if the dev has ever put in cleanup fetish for his games. It could've been added with this update, but maybe that's another rung on the ladder of progressive depravity that we've yet to reach?