Risa has been friends with her neighbors, the Takeuchi brothers, for a long time.
Naturally, they were both into her.

Eventually, Risa married Seiichi, the older of the two brothers.
The younger brother, Ryuji, acted like he was happy for his brother, but a massive inferiority complex was simmering inside him.

One day, Ryuji hears from Risa that, due to Seiichi being infertile, they are unable to have children.
Until then, Seiichi, the golden child, had never experienced any setbacks in life.
If his first taste of failure came at this age, there's a chance he'd never be able to recover from it.

So, Ryuji makes a suggestion to Risa.

"I'll get you pregnant in place of my brother."

[Edited from Steam]