
I thought if mated omegas have sex with someone who is not their mate heir body will start to hurt (Im assuming. Aiji will or already has penetrated Towa)

Yes i remember it, the omega throw's up and inside his body ached when the other alpha rape him this is it http://www.mangago.zone/read-manga/remnant_kemonohito_omegaverse/uu/remnant_kemonohito_omegaverse-chapter-1

Other stories have the omega vomit if he/she tries to have sex with someone other than mate while in heat, but when not in heat, it's fine. Just harder to get pregnant when not in heat. One of reasons I normally hate omegaverse. Alpha can screw up the rest of omega's life and it's just considered normal

As far as I know, the body of an omega changes after the bite so it will not accept others and it makes them sick if someone else penetrates them, but Towa didn't penetrate Aiji

When Omega in heat form a bond with an Alpha the contract is sealed. To avoid the Omega having a kid from other than their bonded mate their body reacts to reject them. The Omega will feel sick when penetrated by another Alpha or Beta (because the pregnancy level is higher than Omega)
So in that case the body only reacts when penetrated and not when doing the penetration?
The Omega is stuck with their bonded pair, but the Alpha can have an harem.
In another omegaverse it is said that only the Alpha can cancel the bond.
Some authors likes to change some rules of the omegaverse world to their likings so you can encounter some differences with the usual rules ( ̄∇ ̄")
So it's normal to get confused if we are used with 1 rule
I thought Matsuo had a wedding ring http://iwebf.mangapicgallery.com/r/newpicIink/tadaima_okaeri/11/fb178ed502ad287bbe548ddfa955ef0b.jpeg but looking again that's not the ring finger so the yuukixmatsuo ship is still on haha
Same haha. I was like HUH he's married..? Nuuuu... -noticed his fingers- Huh he isn't wearing left ring finger but his middle finger. Phew. XD