In the sexuality domain, are "heterosexual" and "straight" exact synonyms?

Even though heterosexual and straight are interchangeable in most contexts, there are times when I find myself wanting to distinguish between an attraction to the opposite sex which is exclusive and one which isn’t exclusive.

In my own experience, straight certainly means “exclusively attracted to the opposite sex.” Thus, a “straight” woman is attracted only to men and never to women. It would therefore seem to me incorrect to describe a woman who has been involved with both men and women as “straight”.

But couldn’t one rightly describe such a woman as heterosexual?

Given the literal meaning of that word’s roots, I feel as though one ᴏᴜɢʜᴛ to be able to use heterosexual to mean someone “attracted to the opposite sex” without implications about lack of attraction to the same sex.

The same thing goes for gay versus homosexual: one ᴏᴜɢʜᴛ to be able to use homosexual to mean someone “attracted to the same sex” without any implication about a lack of attraction to the opposite sex.

The Question

Is this idea backed up by usage? Do some people, sources, and so on make this distinction in meaning? Or does English not have an easy way to express what I’m talking about here?

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.

In common usage (which at the end of the day is what matters - we communicate to get a point across), heterosexual and straight are synonymous. That is, if a person is described as either, they are assumed to be exclusively attracted to the opposite gender; if a relationship is described as such ("He's in a heterosexual relationship right now"), generally it only implies that this particular relationship is one between a male and a female.

In terms of expressing attraction to one gender without further implications: "gynephilic" and "androphilic" and/or "gynesexual" and "androsexual" are words that are often used, or simply "men who have sex with men" (edit: note that this refers to action and not attraction, which appears to be your intention) as has been mentioned, since it makes no mention of sexual orientation. However, in conversation it's probably easier to just say "he likes girls".