Sorry for the unhelpful title, that's probably why I cannot find this.

So, the game tends to give a female Sim her husband's last name regardless of who initiates the marriage (who is under the wedding arch first). However, I offer have female legacy founders, and I want the husband to take her last name. I used to have a mod that made it so whoever was under the arch first was the one whose name was taken. I'm having a hard time articulating this... Did it sound familiar to anyone?

No - the default is that the last name taken is from the one who proposed the marriage. If you have your male sims initiate the marriage, then they will have his name. Or you might have the so-called 'traditional' marriage hack which forces the couple to take the male's name. Personally, if I want the couple to have different names, I just use the batbox or whatever (SimBlender?) to change the name once again when they're married.

In an unmodded game, the sim who is active when you click on the arch to get married, or who initiates the propose/get married interaction if not using an arch, will always be the one who determines the last name.

If this isn't happening, then you have a mod that's changing it. Get rid of the mod, and you're home free.

Maybe this is what you are looking for ?

I'm not sure if the insiminator, inteen, ACR, or Same Sex Pregnancy mess with whatever the game default is (those are the main ones that I've considered essential to my play style, so I honestly cannot remember the game without them). I think the like Rosebine pointed to is the one I was thinking of. Thanks for all the help!

Inteen and ACR certainly don't do anything about names. Insim is, I think, literally just Debug Mode on an object (or sim or whatever) - nothing but existing game code. Can't speak about same sex pregnancy but that just wouldn't make sense would it?

I do have Inteen too, and it is just as it is supposed to be, the last name will be of the sim who proposed. But I remember downloading what I linked also...it is not in my game anymore though. The reason why I had downloading this remains unclear...lol

Rosebine's link to the mod by Marhis is for choosing a last name. It will ask if you want the last name of the pictured sim, or not. A box will pop up showing an image of one of the two sims, read it carefully to choose which one's last name is used. I've been using that mod for a long time with no problem. (:

Ooh that is useful Duine, might grab that myself.