Some sexual yaoi manga drawings drawn by mio, only done for laughs, for example a suggest blowjob (hand on the head) but nothing explicit shown at all.
Some sexual innuendo, very suttle and infrequent, although in a lot of episodes.
Sakurai finds a porno magazine/dvd and looks through it and finds it "very amazing" it is not shown, only the cover that shows a woman in a bikini
Note: there is no fan-service in this show, it just has some sexual references rarely.