Sex (that is, gene-swapping) is necessary for adaptation, so we can assume that it is more or less universal among aliens. Gender dimorphism, however, is only necessary when there are substantially differing sex roles, and whether this is the case will depend on the methods of reproduction used. We can suggest that the various races don't appear to have gender dimorphism either because the genders have similar enough roles that they aren't visibly different, or that they don't have different genders at all and are hermaphroditic.
Organisms that reproduce by fission or budding, or via the release of spores don't need genders at all; they can be hermaphroditic. The only race that would fit this model is the Gardeners, who are in the forthcoming DLC.
Egg-layers can benefit from sex specialization, as producing eggs requires more resources than producing male gametes, but there is often little dimorphism in Earth egg-laying species (in reptiles, fish, crustaceans and non-social insects, males and females can be hard to tell apart), and it's not strictly necessary; for example, each sex partner could contribute half of an egg instead of an egg and sperm.
Organisms that have the most use for sexual dimorphism are those that give live birth, as it is the most biologically expensive for the female. Because of the hostile environment of the Yoral homeworld, I think they will need to give live birth to have offspring that are well-developed enough to survive. There are some hermaphroditic Earth organisms where the sex partners fight to determine sex roles, and the winner impregnates the loser; this seems appropriate for the pugnacious Yoral.
The most extreme examples of sexual dimorphism are eusocial species that have dedicated "queens" that are responsible for all of the reproduction, but we don't currently have any representatives of this type in SIS. This is kind of the go-to cliche for SF writers looking for an "alien" reproductive system based on Earth examples.
It has already been established that the Gremak are egg-layers and that they abandon and occasionally eat their young, so they have no need for a specialized nurturing gender. The adversarial nature of their culture makes sex specialization and cooperation problematic, so they would be good candidates for being hermaphrodites, with each sex partner being able to go its own way after sex and and lay eggs, with neither being dependent upon or having an advantage over the other.
The Orthin have hard exoskeletons and so will probably need to be egg layers. The rational and egalitarian nature of their society does thematically lend itself to a system where each sex partner produces half of an egg, and they are joined in a completely non-sensual external process.
The Phidi are highly cooperative, and so some sexual specialization makes sense; that their society is also extremely hedonistic also lends itself to complex and changing gender roles. Phidi might start life as females, converting to males as they age (perhaps after giving birth or under special circumstances), and one could imagine multiple specialized intersex roles. (Fish people or not.)
Reptiles tend to be egg-layers with little sexual dimorphism, so we can imagine that the Asdhar have males and females but there there is little visual difference between them, and that they do not have different social roles, whether they lay eggs or give live birth.
The Tinkers, of course, have so radically departed from their natural reproductive cycle that the concept of gender no longer has any application to them.