Hmm I see why you think so, but I don't think that's really true. I tried all the different options/variations in that scene on my sharing save. The MC is the one who told her to come over and initiated it. Also, the MC has been the more dominant one in their relationship, and cuckolds aren't dominant. If she would have come over to him, and spit the cum into his mouth, or was the one who initiated it and told him to open up, then I'd agree with you.
It's definitely an out there kink, in my opinion at least, but I wouldn't call it cucky though, the MC just has a fetish that most are likely grossed out by, which is fair enough.