Jeans started off as menswear. Straight legs have been around forever, but have varied in how tight they are worn. Around 45 - 50 years ago, I remember that boys would have their mom take in (peg) the legs because skinnies weren't sold. I guess straight legs must have been loose enough to fit boots underneath until bootcuts were invented!
For women, I would say that straight legs are definitely modern classic, and that bootcuts have more recently become modern classic, what with the variations of baby boots, flares, and wide legs.