Perhaps based on her own (a-hem) experience, Senpai refrained from looking for the new couple after they (a-hem, again) disappeared (Page 3), realizing they needed "special training" to control the expression of natural urges. And in all fairness, no normal person could blame Tacchi for 'reacting' in the way he did on seeing Shimizu filling out her one-piece racing suit as she did ... one might speculate, if she had worn a two-piece, he probably would have been unable to "pull out" in time ... and she might have given her consent to "stay inside" (say-no-more).
It was kind of a surprise that he came outside after she told him to cum outside. Normally after such a line they come inside like there is no tomorrow...
"When me and my girlfriend change into our swimsuits"...We have sex?
I figured that would be the best way to complete the sentence the title started.
That ending though. Well hey, you know what the solution to that problem is? Go another round!
Just keep going at it until the boner goes away...Although since he is a Hentai Protagonist, that might take quite a few rounds?