The secret was:
Cause the chances of pregnancy between an Omega and Beta is pretty low...
People presumed (including Maki) that Iwa-chan is an Alpha cause he got Oikawa easily pregnant.
But Matsukawa knows that Iwa-chan is really a beta...
Iwa-chan asked Matsukawa to keep it a secret because now that he is married to Okaiwa, he must pretend to be an Alpha....(maybe in order to protect him and prevent people from unnecessarily marking Oikawa)
Look at this page : it says here that Iwa-chan will 4ever pretend to be an Alpha
http://www.mangago.me/read-manga/haikyu_dj_tanoshii_jogoku_wo_irodoru_nara/uu/haikyu_dj_tanoshii_jogoku_wo_irodoru_nara-chapter-1/30/
Also Iwa-chan knows that Matsukawa is an Alpha so Matsukawa also wants to keep it a secret between the two of them...
Ah!! Thank youuuu now i understanddd (⌒▽⌒)
Princess_bubblegum sensei, I have a question (raises hand)
Wounldn't it be hard to keep up with the ruse since the Oikawa will still have he's heat also, do you have any idea why matsu wanted him being an alpha be kept a secret?
I’m not sure why he wanted the alpha part kept a secret but the heat can be kept secret with the pill suppressors - at least most of the time I think.
thank u sensei finally got it
The secrettt???