@nyuutop
One angle by which it does make a lot of sense is if you consider her daughter's ego. Mom's not as excited and outburst-prone - the kids get that from their dad.
It's not necessarily that Tsuki WANTS a threesome with her daughter's boyfriend, but rather, she's seeing herself as irresistible. Despite all evidence to the contrary, the slightest lack of context sends her spinning into "he can't control himself, he's helpless against my charms" territory. Him not being interested is simply unthinkable to her.
Think about who else you know that jumps to these head-inflating conclusions...
Like someone whom on being told she has good affinity with Sakurai catapulted her mindset into "he can't live without me!" and "I'm pretty much an enchantress!"