At least he was alone in the shower when he did this.
You've seen Guy Standeven in something because the man was in everything.

For all the times Ive seen this film, this past TCM or MGM showing of it was the first that the meaning of the soap hit my mind. 10 times Ive watched this, got every subtext and nuance but until now missed the soap. Dam

Seriously? It flashes to his grandma penetrating him as a boy by giving him an enema. It's very explicitly showing Joe associating being raped by those guys to that moment of dear old grandmother doing her deed to him. Pretty clearly spelled out for the viewer.

It can be interpeted in two ways. When they had Joe over the car hood or trunk, you could tell they were pulling his legs apart. Now whether that was to sodomize
him or to help keep him from struggling and pulling free its up to your own understanding.

There is a flashback scene in which Joe is sodomized and Crazy Annie is raped

Joe gets sodomized and Annie raped; but not Joe? Yes, dropping the soap in a shower, can be taken in context of a humorous or even homophobic nototion of getting jacked up the whatsy by another guy. In all due respect though, who hasn't dropped a bar of soap in a shower and then had to bend down and pick up the slippery wet thing? Yes, people can read whatever they want into this; but ultimately, all it means, is that Joe dropped the soap while singing in the shower and had to retrieve it.
Exorcist: Christ's power compels you. Cast out, unclean spirit.
Destinata:

And you think dropping a bar of soap is some trigger-alert of whats to come. I will have to rewatch the film and see exactly how that rape scene plays out
I can imagine filmmakers today being very pensive about every frame they use in their projects due to young-audiences who may find some SEXUAL "theme" in everything, unfortunately.

Have you even seen the movie? There is a flashback seen that shows him attacked by a mob and sodomized.

'There is a flashback scene in which Joe is sodomized'
Does the flashback depict him being anally raped, or are thinking of Deliverance? Or is it fun and titillating to imagine handsome Jon Voight in these fantasies so can you say it "foreshadows " events to follow. I noticed in our modern-era, viewers think sex is incorporated into everything, which is disturbing and also "thematic"juvenile. Yes, I'm sure you're really getting your rocks off with your co-posters over it.

You also ruin the much-needed notion that two men can care for each other, without being gay. We already have that stigma in society already; you're reprehensible by reinforcing it. Perhaps your only use for people is sex-related.

Yes, that's a good point.thematic foreshadowing. I like it.

In a way, it's thematic foreshadowing (Joe is unknowingly about to be, ahem, 'screwed' in every sense of the word), but I think really it's just a joke.