
Please, someone has to explain to me why the senpai stretches him using a condom on his fingers but then fucks him raw ┑( ̄Д  ̄)┍

Well normally you will use either rubber glove or condom. Reasons for that usually are; edges of the finger nails can scrape your partner or it feels more clean for both parties (no panic for the poopie traces left in or nasty dirt under fingernails which you see so often). Although that is no issue if you clean inside first, but I think I only remember one yaoi ever illustrating that factor.. I think it was Elektel Delusion and Shunpei cleaning it all. Also the sensation is different with the rubber. At the end of the day, if I were to take someones fingers in, I'd rather they either wash em good and clip them nails or use something in between. :P
OMG i can smell the romance already ╮( ̄▽ ̄)╭