My guess is that he was raped and he spread the rumors around so that his raper (who I guess he was somehow in love with) didn't get badmouthed.
Just my guess I'm not sure myself (￣∇￣")
My guess would be preservation either he was protecting himself or protecting other people from the rapist... Generally in most circumstances like this admitting that the victim was raped would be prove to be hard since it's a very and serious topic plus one would be sacred as well as embarrassed about the topic itself plus not mention a guy getting raped is seriously uncommon so there is a possiblity that no would believe him as well.. not to mention the rapist had a girlfriend then this would leave other people to think as to why he would tajima !!!!
The next theory is that like I said earlier inside tajima would have felt that saying that that upperclassmen raped him he felt maybe no one would believe him and he would be put in a position worse that he already is in so may now that he was falling down the black pit he decided that maybe he wanted to take the rapist with him and spread rumors that he cheated on his girlfriend with tajima so that upperclassmen would also be shunned but other people came to know what kind of a person the upperclassmen really was..
Well so these are my theories but frankly I could be wrong but the fact I'd the matter remains that tajima was raped in a place that he loved the most ... It's a experience that not many of us can relate to wether he did what he did was convince himself that it qas consensual so he could cope better or he just wanted to get back at the senpai we will never know
someone help. i dont understand the last one. was senpai raped in the room? why did he spread rumors abt himself?