I do not want to sound like your mother but you should not play in the rain. I've ridden in the rain many, many times ~ hardly ever by choice.
As previously stated the grease in the middle and the first bit of rain brings oils, fluids and other nasties up over the water for a bit - before they get washed (more or less) away. Painted stripes, as you have already realized, get slippery when wet, as do manhole covers, steel grates and the like.
When you lean a motorcycle a force vector of more than 1G presents itself to the road on a contact patch of rubber that is small to begin with and even smaller as you lean. Centrifical force is trying to right the bike while countersteering or a lazy lean is opposing that force. It all comes together where the rubber meets the road as the cliche' goes.
Anything between the tire and the road that reduces grip reduces the ability of this all to stay together. If you are at the outer edges of the envelope which, by the way, is difficult if not impossible to quantify when rain is introduced, you can lose it with so much as a wet leaf, a coffee lid, the guts of a dead animal, a painted stripe, etcetra. In other words, It can go form seemingly just fine to AF-ed Up in an instant.