I get that you are saying "cosa is feminine" so shouldn't you use "la" as the feminine pronoun to replace "thing" IMHO this is an overly narrow view of what we are trying to do here. The Spanish sentences should reflect what is most often said in Spanish, and the English sentences should reflect what is said in English. We are not looking for literal translations. The reason LO is used in Spanish is not because the speaker is replacing something masculine. LO is used as a neuter sort of wildcard pronoun which frequently substitutes for an entire concept or something said previously.