I have come across the same abbrev in 1841 census M.L. and F.L. or it could be M.S. and F.S. It does not relate to gender because I have a Mary Batty aged 60 who is M.L.
Welcome to RootsChat, great place for help with family history.
I think your Mary BATTY in 1841 was with a Joseph BATTY aged 65, and that the enumerator has recorded Joseph's occupation against Mary rather than against Joseph. I think you will find that the enumerator wrote "M.S." and that is the abbreviation in 1841 UK census for Male Servant.
I attach snip of part of one page from 1841, Salford to show Joseph without an occupation, and further down the page there's a Thomas as " M.S."
And further down the page (but not in the snip), in Thomas' household there's a lass named Sarah, clearly an S and not an L.