What does Engels leave out?

Engels assumes that all relationships are heterosexual in that monogamy and sex love is only for the purpose for procreation.  (Similar to the argument that if sex is only for procreation, then why are college students engaging in it?)  In limiting individual sex love only in relation to procreation, it is impossible to engage in any other relationship that is outside heterosexual means.  Engels therefore relegates sexuality into two binaries, one being male, the other being female and the sex roles that come with them.  By not considering sexuality to be outside anything other than procreation, the preferences of gay and lesbian individuals are ignored.  By creating the binary, Engels ignores those who do not feel they belong to either category.  Esther Newton addresses the binary created in gender roles when she points out there is a third gender.