to the practice of assuming
under the assumption
Heterosexism is different from homophobia because it is not as much of a fear of homosexuality, it is an assumption that anything but heterosexuality is different from the norm. This attitude immediately places all GLBTQQ community in a category of "otherness."
Pretty much everything in society is heterosexist. Even those people that believe that homophobia is wrong often have a problem with the idea of heterosexism. For example, when a Women's Studies professor is discussing the impact of eating disorders as being partially a result of women trying to fit the idea of what they think men want because of advertising, she is making heterosexual mating games normative and not only claiming that this is the "normal" reaction and a lesbian reaction would be "abnormal" (though not necessarily bad), but also completely ignores the lesbian or bisexual experience of eating disorders.
Usually not an intentional attitude. It takes a lot for anybody, including feminists to be convinced that assuming that heterosexuality is normative is harmful, let alone that it's not true.