I recently decided to wipe the slate clean and get rid of all my custom content and downloads, and uninstalled and reinstalled my Sims 2 game. I have all the expansion packs except Bon Voyage.

Anyhow, I created a married sim couple in CAS and moved them into a lot, and they had a daughter who has now reached adulthood. I created a male adult sim in CAS to be her partner, and moved him in with the family. I had them interact, lots of romantic interactions, and pretty soon I had them engaged and the female sim wanting to get married and throw a wedding party. So, wedding party is in full swing, all the guests have arrived and are having a good time... and they won't get married. They can't interact with the wedding arch at all, and when I click on one whilst controlling the other, they can't Propose... Marriage, same if I swap which one of them I'm controlling.

Both of them are Adults, not Young Adults, and they're not in a Uni neighbourhood. Neither of them are married to anyone else, or have any other romantic relationships with anyone else (nor have they ever had any romantic relationships with anyone else previously). Both of them have 100 relationship score for each other both in Daily and Lifetime relationship bars.

Can anyone help?

Is the wedding arch on a residential lot and not a community lot?

Oh wait, do you have Inteenimator in your game? If so, then that might cause a glitch. But if they are already engaged, then it rightfully shouldn't. I had a similar issue but in my case, one of the Sims lived elsewhere and had a home business. I'd try maybe moving just the two of them out, or just one of them out into their own place and trying the arch sans wedding party first. If that works, unsave and throw a party, etc. If the arch doesn't work out, then it might be a game glitch since you don't have any hacks. I'd guess using SimPE or the Sim Blender (VERY safe and good little object) to get their relationship to Spouse is the only easy way.

The wedding arch is on a residential lot.

I can now report that the problem is now solved. I'm not entirely sure how. The next time I went into the game onto this lot and family, in the middle of the wedding party, getting married was now an option! It could have just been a temporary random glitch I guess.

Thanks for the suggestion anyways, it could easily have been one of those, so for anyone else who has the same problem, it's worth checking those out.