In Gensyoko, it's perfectly legal to have more than one wife/husband. The only thing that bother me is Suika/Yuugi child. Shouldn't she have three horns?
And then there's these facts: Suika is too small to bear; Patchouli is too sickly, so she could die during childbirth; And Cirno... Well, she's the strongest!
By virtue of logic, Yuugi was the one who got pregnant of all three. There's no other explanation, guys.