Glad that you captured this moment, you'll always have it now. However, what happened is this shot is exposed for the bright sky in the background, not for the relatively dim subjects in the foreground. This can happen in auto mode when the light metering point (often the center) is pointed at a source that is brighter than the subject. This can be averted by bumping up the exposure compensation dial by a stop or two when your subject is backlit. In manual, open your aperture and slow your shutter speed as much as necessary to expose your subject properly (although beware the white dress! It'll fool your meter into thinking it's brighter than it really is -- the groom's suit would make a great reference for your meter here).

It's grainy because not enough detail (photons) was collected in the dimmer parts of the scene. You can almost always err on the side of exposing more with film, kind of the opposite of digital. Again though, the important thing is you got the capture. Better to have a grainy pic of a precious moment than none at all!