He has a counter proposal

"However... I have a counterproposal," the husband said.

​You leaned forward slightly, surprised. "What counterproposal?"

​"I would like to make your son my official heir," he said smoothly.

​"Why?" you asked, genuine perplexity in your voice.

​He shrugged indifference. "It is no different than me adopting a high-born male child to carry my family name. It secures my lineage with your bloodline directly. You can still sire children elsewhere. Perhaps you can even get Rikako to give you another son later down the line, though there is a possibility that might not happen."

​He rested his elbows on the table, fixing you with a sharp gaze. "We can solidify our ties further by you marrying one of the daughters I had with Rikako. Or, if you simply want a bastard heir in the first place, you could always have sex with my mistress."

​You blinked, a specific name coming to mind. "Ren Serizawa?"

​A smirk touched his lips. "Oh, so you know of her? You fancy that one, I take it? I was told you had keen eyes. But you should realize... a man in my position has more than one mistress. So, how about it?"