For not a lot of money, I think the EF 85mm f/1.8 is a good candidate for low-light, indoor weddings. The 50mm f/1.4 works decently, depending on how much flexibility you have in movement. I have also used the EF 70-200mm f/2.8 L IS USM (orig. version) with great results, but I usually turn to the primes when the light level dictates an even wider aperture. But if I had to settle for an f/2.8 aperture, I don't think I would buy a prime - I'd rather have the versatility of a zoom (EF 70-200mm f/2.8 L IS II USM / EF 24-70mm f/2.8 L II USM or their predecessors).

I find the Sigma 35mm f/1.4 works very well for indoor events - I think it's a great focal length for wedding receptions.