The difference is that it's not "seeing" a murder. It's committing a murder vs watching pornography.
Killing somebody in a video game isn't real murder. Seeing a nude character is still pornography.
Likewise, having sex in a video game isn't real sex. It's still pornography, okay, sure, and..? And..?
That's the point. Many people don't like pornography. So much so that they've collectively grouped together to demand payment processors make changes.
The difference is that it's not "seeing" a murder. It's committing a murder vs watching pornography.
Killing somebody in a video game isn't real murder. Seeing a nude character is still pornography.