Sims can't get married even though both are on same lot

If what you are trying to do is to get your Sims married spontaneously while they are visiting a lot with a wedding arch, for some reason it would not work anymore (it used to)

As a workaround, you can send one of the Sims back home and the get the other sim to invite him/her back to hangout at the current lot. Then, you can click on the wedding arch and the option to get married should work.