Yuuma (omega) purposefully got pregnant by timing his heat while Ren is visiting his place. the reason he did it is because he knew Yuuya and Ren are in love with each other, but Yuuya would never say anything because he's an alpha and has always put Yuuma’s needs above his first, and has already decided one-sidedly that Ren is better suited with Yuuma. so he got pregnant and left after having the baby because he knew Yuuya would never let Ren suffer alone and step up to help, thus allowing Yuuya to be together with Ren as a family. he did acknowledge it was fuc*** up and has regrets about it, but well, he did it with a good intention i guess. and he succeeded because Ren was moved by their conversation so he finally confessed to Yuuya and told him he would never let him go
Spoiler!
imo twins actions are incredibly stupid and what he did to ml honestly can be considered SA.