Hello!

I've been wondering if there is a mod that can give a sim the betrayal or cheating moodlet when their spouse/girlfriend is pregnant with another sim?
I found it kind of unrealistic that a sim will think their signification other is pregnant with their kid without even woohooing with her

Should be possible with NRAAS MC Cheats.

how do you know they did not?

I maybe wrong (or it's some of forgotten by me NRaas settings or else) but if the sim in relationship give birth to the child conceived by ehm... 3rd party involvement, there's a relationships hit. Still - it seems like for them it's not a big drama, unlike an actual "action". Apparently they believe in "what you cannot see, cannot hurt you".

I found that weird. I had a sim reveal to her boyfriend she's pregnant (by another male sim), and he just... "Aww, that's wonderful!" *pat pat*
Uh... she's giving you a Kukucksei. Aren't you mad about that? And I didn't notice a relationship hit.
Though, if I recall correctly, that was in the console version of Sims 3...

in my game they get the relationship hit (maybe another mod? IDK).

It's always "mother's baby, father's... maybe".