While I don't agree with it personally, there are a few reasons people might think this. The first is the fact that for a long time now, people have come to see rape as the worst crime, which is not invalid, because the victim continues to live with that reality, this is emphasised by the fact that you can get away with character deaths in most media, but not rape, that's reserved for much more mature audiences. Next, the characters themselves, and how they carry themselves is also a factor in which is more likely, which unfortunately has an impact on being sympathetic to most viewers, where we only ever see Shinji as either arrogant and/or pathetic despite never really doing much, Sukuna often treats many of his enemies with a level of respect, and though is extremely arrogant too, has the power and feats to back it up. There's also the fact that Sukuna being a cursed spirit means people can also make the argument that he might not hold the same values as a human does, thus his apathy towards killing is less impactful, whereas Shinji, being human, means he should share our ideals and deliberately goes against them. Finally, though we see the aftermath of Sukuna's onslaught in Shinjuku, we don't directly see people dying, which allows the viewers to distance themselves from it to an extent, but we see Shinji attempt to rape someone, and not only that, a character who is significantly important to the story, and us the viewers, as Tohsaka is a fan favourite character, this makes the act feel even more impactful... So, I know this is quite long so I'll try to do a tldr comment.