Gay and lesbian folks, when gathered together, do tend to refer to gender-discordant couples as straight (or we presume them to be straight). We typically use straight in the same way we use gay and lesbian and don’t typically (at least in my experience) use the terms heterosexual and homosexual, probably for the clinical reasons described above by @seekingwolf. As I don’t tend to discuss sexual orientation with members of gender-discordant couples, I don’t often use the word straight when talking to straight people, but mean no disrespect when I do use the term in mixed groups.
Words used by any minority, between themselves, to describe the majority may, in fact, be offensive when no offense is intended. It is a quick sorting of “my tribe” or “not my tribe” or “hmm, I am not really sure.” Often such words have been used to describe who is “safe” and who is “unsafe” to the group. Terms like “breeder” are more typically used in the pejorative sense when referring to the “not my tribe” person who is perceived as offensive to the group.
Similarly, members of minorities are able to use words to describe our own that when used by non-members of the tribe are perceived as offensive (e.g., lesbians referring to each other as “dyke” as in dykes on bikes). In part, this is an effort to reclaim the word and to remove much of its sting when used by non-tribe members.
Personally, I am delighted that more and more “straight” folks are beginning to better understand my tribe and to allow us equal rights and participation in socially-normative behaviors (e.g., marriage) and such change may allow for a different vocabulary altogether—one that is affirming to members of the many different orientations that exist. I love the “straight but not narrow” slogan that has emerged as I know many who represent THAT group.