I had my sim ask another one to marry it, and it said no. So I tried a..

You have to have them move in together first.

U just hav 2 do romantic things then you can marry it goes like this there is a cheat and I'll show you get them woohooing and go back by closing the Sims press the button at the end of the iPad or iPhone (not screen ) or slide to a screen were there are no apps on the bottom 3 rows then put 4 fingers on the bottom row and pull up loads of apps should pop up at the bottom of the screen when you see sims press and hold the sims until you c a red circle press it and the sims will go from the bottom row of apps but is still there as a game next you go to settings ,general and date and time pass forward a day or 1 year and turn off wifi then go back to sims and you will find they have finished woohooing and will have done a few romantic things and you will also have loads of money so you propose they get married it worked for me hope it does for you too B) plz comment thank for reading

You have to try a more expensive ring.

They have to woohoo first and you have to buy a more expensive ring.

Follow these steps

1.make sure they live together
2.make them woohoo
3.try to buy an expensive ring the nicer it is the more chance there is of acceptance but NEVER buy the dino vending machine ring it hardly ever works but does work occasionally but still Don't take the risk

Give her a more expensive ring be ause he looks richer

Make sure the like each other first or it might not happen