When you say one toilet, do you mean one set of restrooms (men and women) that will each contain a few toilets? Or do you mean, there's one unisex restroom that has one toilet in it? To me, one toilet for 60 guests is not enough and having the ceremony somewhere with more bathrooms doesn't help much (maybe if you made an announcement everyone should use the bathroom?) How long are you anticipating the reception to be? To be perfectly honest, if I had to go all the way downstairs to the ground level or go to my room, I don't know that I would come back. If it was in the first little bit, I probably would. But after an hour or so, I'd probably just leave, but I'm not much of a partier. If restrooms were available just one floor down, that wouldn't bother me as much.
I will note that I have digestive issues and that has led me to be more aware of restroom situations than others might be (for me, a one bathroom house was a deal breaker - it would bother me too much) so not everyone would feel the same as me. Does the hotel have any advice? Is it usually a problem? Have you already booked it or are you thinking about it? I don't think it's wrong to have your reception there, just be aware that it might become an issue for some people.