What is the girl in lesbian cowboy with Matthew modine?

It appears you are referring to the 1994 movie "The Cowboy Way", which starred Kiefer Sutherland and Woody Harrelson as cowboys, not Matthew Modine. There is no lesbian character in the film.

Perhaps you are thinking of a different movie? If you can provide more information about the movie you're thinking of, I might be able to help you find it.