At some given point in history, organisms must have made a switch to male/female reproduction. Since each sex would have needed to evolve at the same time, this seems like a roadblock on the evolutionary highway.
Not necessarily, Take a hermaphroditic species such as an earthworm. 2 different worms mate with each other the female part of worm one with the male part of worm 2 and vice versa. Given time,a such an organism could mutate to only having male or female reproductive organs and still mate with the hermaphrodite.
There is no better love between 2 people than mutual respect for each other WT Young, 2002
Who gave anyone the authority to call me an authority on anything. WT Young, 1969
Since Evolution is only ~90% correct it should be thrown out and replaced by Creation which has even a lower % of correctness. W T Young, 2008