Re: What I think about Loyal Trait and the addition of baby behavior:

@daikoyu
Hi, there
Thanks for the new input and suggestions.

A reward store might be a good idea. But only if the Sim has enough points to gain the trait. If the marrying party is an NPC or a not played Sim, it cannot be gained immediately after the marriage. So I thought that if it was incorporated into the marriage event, even a Sim has no points could gain the trait immediately after the marriage, and the player selected whether or not to gain the trait, thus making the story as the player wants it to be.
However, after reading the comments from PipMenace and thinking about the system built into the Sims4 world, I realized that the Flirty Moodlet is the cause what makes Sims look dishonest, and I was thinking that it makes no sense to link marriage to the Loyal trait. Now I think if Sims gain any trait by getting married, it is not a Loyal trait.
Do you think that even if the system were to allow the player to select whether or not to gain a trait (another trait that is not the Loyal trait, of course), it would not be the story the player wants?

In your comment,
"So, a lesbian could marry a man because on the storyline of the writer women are forbidden to have weddings with other women's and the family decision that she need to marry a man."
(I think I saw a female Sim having a wedding with an another female Sim in the PV of the My Wedding Stories pack, but can't a female Sim have a wedding with a female Sim? Or does it mean about a story setting as envisioned by the player? I don't have MWS.)
I'm not sure about above part, but I understand that "exploring romantically" should not be changed in marriage, and wrote so in my last post.

Sims are being dishonest because of Flirty Moodlet.
The Loyal trait proposal I wrote in my OP is forfeit. Because that to being loyal is not equal to being faithful.
If the players want a storyline where the Sims do not behave dishonestly after marriage, I think the trigger would be marriage.
But how do we control the behavior that Moodlet triggers? And if you have to trigger a single event to make a change, it seems like it would be a very cumbersome design. I don't think this is likely to happen.
So the days of spousal Sim's dishonest behavior continue to haunt me...

Two stages of early childhood.
Yes, one possible change would be to add a stage.
But adding a new stage would mean that the aging system would have to be modified.
I can welcome the introduction of crawling, but I don't want the aging system to be broken.
How frustrating.
Thanks for your opinion again.