I recently took a trip around the Milky Way, and visited several planets with alien civilizations.
On Ryeleh, the sentients go through a female puberty in their late childhood, and later in life go through a male puberty.
On Maduro, the sentients are sexually undifferentiated until early adulthood, instead of differentiating in their embryonic phase. The sexual differentiation is driven by a formally chaotic complex of hormones, so even the Madurans do not know their eventual sex.
On Betamax, males and females are physically identical, unless their internal organs are scanned.
On Venn, the sentients are infertile offspring of the infrequent mating of two smaller-brained species.
On Fallon, the males are small brainless worms the females keep as pets. A compatible male is absorbed into the female body and becomes essentially just another organ.
On Earth, sexual differentiation is not absolute, and several percent of the sentients are born or develop with intermediate or simply atypical characteristics. The sentients often conduct largely unsuccessful surgeries in an attempt to hide the biological reality.