Why reproduction settles on two cell sizes
Sexual reproduction in almost all complex life works by merging exactly two specialized cells to form the first cell of a new individual [4]. Those cells are not equal partners. Across the natural world, the smaller "male" cells — sperm, pollen — are typically numerous and mobile, while the larger "female" cells — eggs, ovules — are usually fewer and immobile [4].
This size difference is the anchor for the whole system. A cell packed with resources to support a developing embryo is a fundamentally different object from a stripped-down, mobile delivery vehicle, and each is optimized for a different task. Because the two roles are so distinct, it makes sense that they get divided between two classes of individuals, each with dedicated reproductive anatomy and secondary physical characteristics that support its role [4]. One way to frame this is that an animal's sex is not really about which cell it happens to make, but about which cell its body is built — by evolution or development — to make [5]. Under that view, male means having some part or process whose biological function is to produce sperm, and female means the equivalent for eggs [5].
Two sexes does not mean every individual fits neatly
It helps to separate two questions that sound the same. If "sex" means the broad reproductive pattern of a species — reproduction through the fusion of two differently sized gametes — then yes, there are two [1]. But if "sex" means sorting the individuals within a species into two tidy groups, the answer is often no, and it has to be checked species by species [1].
In our own species, the typical primary and secondary sex characteristics usually line up with the egg-versus-sperm distinction, but there are many naturally occurring developmental variations that do not line up that way [1]. That is a statement about individuals within a species, not a contradiction of the two-gamete system. The paper making this point also stresses that gender is a separate, sociocultural kind rather than a biological one, even though the two are often experienced as tangled together [1]. Keeping the species-level pattern and the individual-level variation in separate boxes avoids most of the confusion.
Why the two sexes often differ in body and behavior
Once there are two sexes, evolution can push them in different directions, because the costs and payoffs of reproduction are not the same for each. A clear example comes from a jumping spider that differs between the sexes in color and shape but only weakly in body size. When food was restricted, males and females responded in much the same way: both took longer to develop, ended up smaller and lighter, spent more time out of their nests watching for prey, and caught prey more successfully [2]. Yet even under identical food conditions, females developed faster, were smaller as adults, were more likely to stay outside nests, and had higher hunting success [2]. The authors read this as a possible balanced situation, where the two sexes show equal flexibility to environmental stress but still carry built-in differences [2].
Sex differences can also show up in the immune system. A modeling study tested the idea that sexually transmitted infections shape how much each sex invests in immune defense versus reproduction. It found that higher susceptibility favors tolerating the parasite, while higher virulence favors resisting it, and that the different reproductive strategies of the two sexes lead to differences in immunocompetence — mostly higher in females [3]. Importantly, the same study concludes that these sex-specific immune strategies are less predictable than once thought, which helps explain why sex differences in immunity are not universal across animal species [3]. In other words, having two sexes sets up the possibility of divergence; it does not dictate one fixed pattern.
So is "two" the right answer?
The most defensible summary is that two sexes are the standard because two gamete sizes are the standard, and the two size classes map onto two reproductive roles [4][5]. That is a claim about the architecture of sexual reproduction, not a claim that every animal, or every individual, will present as a clean either-or [1]. The empirical work on spiders and on immunity shows the same theme from another angle: the two sexes can be strikingly similar in some responses and clearly different in others, and which is which depends on the species and the trait being measured [2][3].
A useful comparison is a lock and a key. The system needs exactly two complementary parts to work, but that says nothing about how uniform the keys are, how many slightly different locks get made, or how often a particular lock and key fail to pair up. The two-part logic of eggs and sperm is what makes "two sexes" the default answer; the messy, species-by-species reality of bodies and behaviors is why the default is not the whole story [1][4].
Sources used in this answer
Are There Two Sexes? Yes and No, But Mostly No (and Gender is Something Else Anyway – More or Less)
Sex as a species-level pattern of fusing two differently sized gametes yields two sexes, but sorting individuals within a species into two groups is often not possible and must be assessed species by species; gender is a separate sociocultural kind.
Two sexes respond equally to food restriction in a sexually dimorphic but not body mass dimorphic jumping spider
In a jumping spider with sexual dimorphism in color and shape but weak size dimorphism, food restriction produced similar developmental and behavioral responses in males and females, while some sex differences persisted regardless of food condition.
Sexually Transmitted Infections Select for Different Levels of Immunocompetence And Reproductive Efforts in The Two Sexes
A model of reproductive and immune investment against a fertility-limiting sexually transmitted infection found that differences in reproductive strategies between the sexes lead to sex differences in immunocompetence, mostly higher in females, but these sex-specific immune strategies are less predictable than previously thought.
Two Sexes
Sexual reproduction in almost all complex life involves the merging of two differentially specialized gametes, with small mobile male gametes and large immobile female gametes, and these two sex roles are divided between two classes of individuals with dedicated reproductive anatomy.
Sex by design: a new account of the animal sexes
An animal's sex depends not on the kind of gamete it is capable of making but on the kind it is designed to make, where design is understood through evolutionary or developmental selection processes.