I've been playing a completely clean game. No mods. Also, I have just installed my games because my hard drive broke beyond repair, and I had to replace it.
I'm sorry but I have no idea about it. I wish that I could help you.
It might be because the engaged moodlet has worn off, that's the answer that makes the most sense to me.
Yeah, maybe there's a time limit for when you can throw the party and when the moodlet goes away, time's up.
Just as others say. Yeah, maybe there's a time limit related? I usually throw the bachelor or bachelorette party when they are still engaged. And soon do the party afterwards. Usually I was reminded with other Sims that who my Sims friend to throw the party. Maybe, there's a chance to get the option back if you try to break them up and reunited them again (not recommended but it's worth checking)
Also, I was checking too into old couple who only one of them that throw the party when they was young. They have been together since 57 days. And Bachelor party not show up as a party option. My other Sims who have been together for 23 days too don't have this option when they decide to throw the party now, they are married without do bachelor and bachelorette party before.
I remember. The option still available though when they are newly wed. But, it's not for a long time.
This maybe can be a hint
UPDATE: Ah, yes! I can see it now. It's like what Rainbow Dash said. It has something to do with the moodlet.
So, I make my Sims get engaged, the bachelor/bachelorette party option is still available.
Soon, I make them do Private Wedding. The moodlet "Just Married" appear for 2 days. So, I was tried it again.
And tadaa... The option for bachelor and bachelorette party is disappear. Hope it will help you!