Straight-run merely implies that you’re obtaining the birds “as-hatched.” No effort happens to be built to figure out the sex for the birds.
Does “straight-run” imply that half is likely to be males and half females?
No, certainly not, whenever you hatch chicks, they may turn out 50-50. However you could also have significantly more than 50% men. Or maybe more than 50% females. It may differ a whole lot.