In that case, why is it that all married CAS sims never met and kissed anyone before they met their spouse? And why, if you create children with an unmarried couple, does the game suddenly decide that those children are the child of only one of the genetic parents, and the *mystery sim*?
Your first question is as much a puzzle to me, as it is to you, ZZ. Although I guess they just consider the CAS Sims to have gotten married to the first one they met. Like: we don't need to introduce Mystery Sim, because we have a target for these memories here.
As far as the second question goes... I think there might still be some logic behind that as well. Kids in CAS *must* be linked to at least one parent. If you don't link the parents to eachother as being spouses, the game may assume that the unlinked adult is NOT related to the kids, and therefor introduce the 'mystery sim' to fill the hole.
On a similar note: open a new 'hood and start a new *single* sim family. Now whatever happens, play *only* that one household and let this sim build up a relationship with an NPC like the Mail Delivery Sim. If, at any point, you make this deleviry Sim selectable, you'll see they had absolutely NO memories before meeting your sim. Not even mystery Sim shows up. They *will* build up all kinds of memories about the relationship you're working on, though... like the first kiss, the first woohoo and such. Now when the relationship is at the appropriate level, have your sim propose engagement and marriage the Maxis way, and then check the new household member.... all the memories preceeding the move in have suddenly been erased (the game assumes that since it was an NPC, it wouldn't have memories anyway), and replaced with the well known 'Mystery Sim' ones, except that 'Mystery Sim' is now replaced with your original Sim.
Billy-Bob Farmer in my first LOTL attempt had such a relationship with Tristen, whom he married: her memories after marriage were like this:
Met Billy-Bob Farmer.
Made Best Friends with Billy-Bob Farmer.
Had very first kiss with Billy-Bob Farmer.
Kissed Billy-Bob Farmer for the first time.
Got engaged to Billy-Bob Farmer.
Got Married to Billy-Bob Farmer.
Everything that she had *actually* experienced before the marriage was *gone*!!! But at least the game did not introduce Mystery Sim, because it now *did* have a suitable target for those events.
Maxis, in their own way, has tried to create some form of consistency... they failed horribly, of course! But the attempt was admirable, compared to many of the other things they managed to screw up.