While on the surface this is act of love... it's rather peculiar. It's a Fallout 3 mod created for a single purpose, and a single person: to ask a Fallout 3 player player to marry the girl who - with the help of the Fallout 3 modding community - made it. The full story begins here on the Bethesda forums, and continues here. To cut a very long story short: a video of the marriage proposal mod being played is below. And he said yes.
Thanks to Alex S for the tip.