if the uke is the father then who is the mother? were they married before?

Just a surrogate mother

No marriage. He used a surrogate mother.

ohh then he it was actually his sperm??

i mean who's sperm is it??? The uke's or the seme's??

So far, It was revealed that it's the Seme's sperm. (unless there's a further plot twist about it in the future).
The confusion of whose baby it belongs to is because of the power play by the Seme behind the process.
The uke was the one who actually really wanted to have a baby. But then the seme was having somekind of a dispute with his family over whose blood-line heir to take over the family's business. His greedy family was using any means to prevent him from having an offspring, so he had to do it in discreet to get off from their radar. That's why he ordered his secretary to switch the fertilized egg with his own and freeze the uke's egg instead. He had the uke raised his son for several years to avoid his family from knowing that he already had a biological son who would take over the company in the future.

thank you so so much ヾ(☆▽☆)ヾ(☆▽☆) that was a lot of information. Thank youuuuuuuuuuuuuuuuuuuuuuuu