The point is you were not thinking about them when you left it up.
The 'little-things' matter a great deal to women, not-so-much to men (so you cannot use introspection to understand.)
So if it is so little effort and shows that you were thinking about her, why not do it?
Generally, it's not that they have now have to do X - it's that you were effectively ignoring them and being thoughtless or uncaring.
You took a vow to cherish her.
The up vs down crap is missing the point; perhaps we should discuss which end of the egg to crack first?
Big endian vs little endian?
You are a bleeding idiot if you sit in a toilet.
You are immature if you further blame someone else for your mistake.
She is also missing the point if she /expects/ the seat to be down.