straightness has 4 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are curliness, crookedness.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
freedom from crooks or curves or bends or angles
trueness of course toward a goal
“rivaling a hawk in directness of aim”
a sexual attraction to (or sexual relations with) persons of the opposite sex
Synonyms, antonyms and the broader and narrower relations for straightness come from Princeton University’s WordNet. Full sources and licences.