Because its only implied not shown explicitly.
Its implied during the random event where the player is tazed and then wakes to find themselves on the train tracks wearing only a pair of underwear. and that is also supported by the last few words the player hears just before the screen goes black.
Neither T nor Floyd have clothing removed, and nothing that would indicate rape. Sadly, I can't say the same for Mr Raspberry Jam.
It's a simple case of people reaching in a desperate attempt to see something that is simple not there, and never was.
Nor is there any dialogue that would suggest that Trevor was raping him
Top that off with...
If you stand by him after he gets out of the bed and listen to Floyd, he sounds like a man who is completely terrorized, rambling things to himself, even sobbing.
Terrorized, the same way that he terrorizes Ron.
Ron is so browbeaten by Trevor that if you walk up and have a conversation with him while playing as Trevor, he insults him and Ron just goes along with it and agrees.
If you play the hangar missions in GTAO and fail, Ron calls you up and says "I'm sorry it must have been my fault somehow". When it was obviously the players fault.
All of this is a direct result of Trevor's treatment of him, and the same can be seen in Floyd.
Trevor seems to use the fear he instills on people to his advantage