Well, what do you mean rain? It rains most places on this planet. Some places it rains every day, some places once evry couple of years. Some places it rains in torrents, others it rains lightly.
I'd say if I were to bike in the rain, I'd be well prepared with the proper equipment. I'd ride using the 12 raindrop principle (of my own design). It's rather like the Beaufort Scale. Select an area about 1 metre square. A puddle is excellent, a square metre of grass is poor. If all you have is grass, use a square metre of newspaper.
Observe how many raindrops fall onto the square metre in a second, and the size of the raindrop. If the raindrop is moderate, then I will bike when a maximum of 12 hit a square metre in one second. With the rain gear I have, I will remain dry at that rate. You've got to figure out your own Stokell Scale.