A couple years ago I made a tumblr post on the topic of comparing demonic/angelic possession to rape, a comparison that was made a lot in Supernatural's tumblr-based fandom at that time. I've since deleted my tumblr, but decided to re-create that post here since it was such a succinct summary of my views.
At the time, Sam's Gadreel possession was a very recent and controversial thing (I don't read SPN tumblr fandom now, but while I imagine it is still controversial it is not nearly as fresh). Comparisons between possession and sexual assault had existed in fandom prior to season 9, but after the Gadreel possession they skyrocketed. The comparison to rape was used to highlight the horror of Sam's experience, and sometimes to vilify fans who downplayed the seriousness of the possession.
So here's my thoughts:
I think that within the fantastic context of the show, possession is worse than rape. I think Sam Winchester, who has been both possessed and raped, would agree with me. Possession is a greater and more horrifying violation. The way we often see Sam's body used to hurt and kill others is particularly disturbing.
That said, even in-story there are a lot of differences between possession and rape. You can possess someone for the greater good (see Sam forcing a demon back into Eunis in What's Up, Tiger Mommy?). You can't ever sexually assault someone for the greater good.
And I find many the comparisons of possession to rape to be examples of fans trivializing a real-world issue for shock value.
I have a disturbingly high chance of being raped, as do large numbers of people I encounter. According to RAINN, in America alone a person is sexually assaulted every 98 seconds. By contrast, I have a 0% chance of being possessed, as does everyone in the history of ever.
And therein lies the most significant difference: Rape is real. Possession is pretend. Piggy-backing on the intense emotions associated with a real-life violation to get empathy and outrage for a fictional character in a fantastic situation is iffy at best and harmful at worst. I don't have a problem with people pointing out that rape and possession are both violations of a person. But any attempt to highlight the severity of Sam's situation by relying on shock value comparisons to real world horrors is not okay.