Spoiler
1) He likes hearing about people's relationship/sex life per shoichi's route
2) He both respect and hates Shoichi. So confirming they were boning would give validation to the hate [though opposite happened]. As well as chance to be spiteful and and bust in.
3) Haruki was never a prude person. not shy about sex unlike Yuuichi. nor does he give a rat ass about anyone's privacy.