Is it ok to stack LPs horizontally?

No absolutely not. Yes they will warp badly 

Actually, its the recommended storage method as long as its your Rick Astley collection. They won't warp!!!!!

Everyone seems to store records vertically yet it's far from obvious just why.

Is it because the printed label is usually slightly raised, and storing flat would push the edges down eventually causing warping?

Is it the same reason people store guitars vertically?

Long term gravity effects?

If so, then I suppose they would have to be stored totally straight up and not at an angle.

Warpage is always the reason behind not storing LPs flat, yet I have never seen anyone give a reason as to why that would be. Anyone?

It has always seemed to me that the only point at which storing LPs vertically may become advantageous as regards to warping is when they are fairly tightly packed on a shelf with firm left and right boundaries.  Otherwise, like someone else said, storing them vertically really means storing them at an angle to the vertical, which could actually promote warping.