They did, just the condom failed. Poor Hye-sung. Other omegaverse stories sometimes show the omega's take birth control tablets as an extra form of contraception too.
The condom didn't fail. He had his dick in him for a short time in him, that's why he got pregnant
There was a short period of time where his dick was in Hyesung without protection. That was enough. This is the case in real life too. The little swimmers start swimming long before ejaculation.
actually he came in him the first time before using the condom like 3 seconds after
Yea no he didn't cum in him it was just precum, which can contain sperm and one can get pregnant from it.
Remember to wear protection kids ;)