Haha... comparing their arguing over Urisugi to arguing about cake... and then they actually argue over cake. Also, nobody likes thinking about their parents doing it, or when they were conceived. This is a pretty universal thing, regardless of genders, as highlighted by Kawashima's face during that story about her mother, and Ichikawa's reactions to realizing she was probably also a product of Christmas sex. Plus that conversation at the end where they're like 'yeah, let's stop talking about this'.
My siblings and I are actually all over the year. Eldest sister was born in March, I (youngest and only male) was born June... second oldest sister was born August, and the sister immediately older than me was born September (oddly enough, one day away from our father's birthday). Also worth noting we're all like 1-2 years apart from each other (eldest was born only 5 years before me), so... yeah...
I'm gonna stop thinking on this, now.