Well I do not know where you live, but this has not been my experience. And yes, I cannot imagine bringing it up out of the blue like you're talking about someone's favorite color, given it's the most instinctually abnormal behavior out there.
Comparing incest to cuckoldry or swinging is like comparing someone who eats shit to someone who was the victim of rape. One is a delicate topic that involves a clear victim, and the other is just someone doing something really stupid. People even use "inbred" as a term for moron. People see the husband involved in cuckoldry or swinging as a victim. At the same time, like I said earlier, it's very uncanny, and people don't like talking about subjects like this. Obviously as the dev of this game, you don't mind, and it's not surprising that your wife is not that bothered about it. But for most people, these topics are just too weird to talk about.
Whereas incest has been normalized, and we feel like we can talk about it (with our non-family members). Maybe because we all find it physically repulsive, but at the same time, there's no victim involved, just two idiots. So yes, regardless of if it's your wife or someone else, they would probably have more to say about incest. But most people would feel more pissed off about swinging/cuckoldry.