Mio just treasures shino because shino is straight. He just doesn't want to break off their friendship if shino knows how mio plays around with his sex friends. Mio just worry about all those when actually Shino already know it all. For me it is not that rushed? since shino has been observing Mio, didn't realized his feeling and that climax when he caught him sex in the cubicle, that time Shino actually wants to hold him. It is a good story for them.
That book is drunk i mean they are already so gay for each other lmao