I certainly don't think that because Mary is riding in a car with other girls it is reasonable to assume she is a lesbian -- if that view is valid, then the fact that the guys who engage in a race with the girls are also riding in a car with no companions of the another gender would imply that they must be gay as well. And what would it say about the lecherous young guy boarding (until Mary shows up)in a house with only an old woman? Some kind of Oedipal relationship? And once you begin this kind of out-of-thin-air kind of analysis, you have to find some kind of psycho-babble explanation for why Mary wants to "play an organ" in a church -- what does she see when she looks at those "phallic" pipes of varying sizes as she plays -- barefoot!-- aha! a foot fetishist's delight! -- in the church! until music reaches a "climax" in tones that are "blasphemous" -- ???!!!
Once this sort of "interpretation" starts, there is no end -- anything can mean anything in the eyes (or imagination) of a beholder. Whether it is logical, or motivated by demonstrable evidence from within the work of "art," or is in keeping with the probable human motivations of its own time period instead of forced upon it by the mores of our own period --- ah, there's the problem.
No, I don't think there's any reason -- within the movie itself -- to see Mary as a lesbian -- she is devoid of emotional connections with people because of what becomes clear at the end of the movie. In her situation -- which she only gradually grasps -- sexuality is of little significance, ultimately of no importance at all.