Queershipping is defined by its fans, therefore, as in-character Rocketshipping. This is mainly brought on by their coordinated crossdressing (although this practice has waned on the show as of late) and their displaying traits associated with the opposite sex.
Unlike Rocketshippers, the average Queershipper does not insist that either of the two are heterosexual; James's alleged bisexuality (at the very least) is an accepted fact among most Queershippers. Speculations on Jessie's sexuality vary, but her sexuality is not the subject of nearly as much debate as James as it is extremely apparent in canon that she is attracted to males, which James is (at least in the academic sense). Despite this, some Queershipping is hinged on the credo that Jessie is a lesbian and James is the only girl pretty enough for her, which does sort of make sense if you look at it in the abstract sense of what the word "lesbian" can mean (any woman who skirts a conventional married life and resists male tyranny, according to the feminist writer Adrienne Rich). This "only girl pretty enough" credo may or may not be tongue-in-cheek, depending on the individual Queershipper.