Straight is a metaphor, and means heterosexual only in a limited set of contexts. Most of the time it doesn't mean that.

• It's 20 miles straight North to the border.
• I'm pretty sure she's been straight with me about that.
• Stand up straight and look straight ahead.
• Honestly, you're such a straight arrow; would it kill you to ignore that stuff?
• Be sure to position it straight up; we don't want to have to brace it.

The metaphor theme is Morality is a Straight Line, which is part of Life is a Journey. The presupposition is that homosexuality is immoral, while heterosexuality is moral, and therefore straight.

Straight is a metaphor, and means heterosexual only in a limited set of contexts.
Most of the time it doesn't mean that.

• It's 20 miles straight North to the border.
• I'm pretty sure she's been straight with me about that.
• Stand up straight and look straight ahead.
• Honestly, you're such a straight arrow; would it kill you to ignore that stuff?
• Be sure to position it straight up; we don't want to have to brace it.

The metaphor theme is Morality is a Straight Line, which is part of Life is a Journey.
The unspoken, but very prominent, presupposition of this metaphor theme is that homosexuality
is immoral, while heterosexuality is moral, and therefore straight.

Straight is a metaphor, and means heterosexual only in a limited set of contexts. Most of the time it doesn't mean that.

• It's 20 miles straight North to the border.
• I'm pretty sure she's been straight with me about that.
• Stand up straight and look straight ahead.
• Honestly, you're such a straight arrow; would it kill you to ignore that stuff?
• Be sure to position it straight up; we don't want to have to brace it.

The metaphor theme is Morality is a Straight Line, which is part of Life is a Journey. The presupposition is that homosexuality is immoral, while heterosexuality is moral, and therefore straight.

Straight is a metaphor, and means heterosexual only in a limited set of contexts. Most of the time it doesn't mean that.

• It's 20 miles straight North to the border.
• I'm pretty sure she's been straight with me about that.
• Stand up straight and look straight ahead.
• Honestly, you're such a straight arrow; would it kill you to ignore that stuff?
• Be sure to position it straight up; we don't want to have to brace it.

The metaphor theme is Morality is a Straight Line, which is part of Life is a Journey. The presupposition is that homosexuality is immoral, while heterosexuality is moral, and therefore straight.