Here are my notes on slugmen biology and reproduction, and how it affects their culture
tl:dr: slugs are not mammals, and slugmen are not a natural race.
Slugmen are hermaphrodites and do not have a biological gender. However, a few (perhaps 10%) do adopt the human custom of having genders.
Slugmen were "invented" by the Archmage Enftebtemang a very, very long time ago to assist him in his endeavors. At the end of his very long life, he revealed to his servants the secret so they could continue their race and his work (keep those damned Krakens from going upriver!)
When two slugmen mate, they exchange spermatozoa and fertilize their eggs internally. After a few months of gestation, the eggs are hatched, about 2d4 per parent. The eggs are then given to the Tea Master, who will bathe them in strange substances and perform arcane rituals for another few months. The eggs will then hatch into a large slug (1-2 pounds). These sluglings will be fed special diets and closely guarded for about a year. At this point, the surviving slugs will be subjected to various tests. The one deemed superior will be selected for a final ritual and series of treatment, resulting in a metamorphosis yielding a small, young slugman, at which point the Teamaster's role end and the parents begin anew. This metamorphosis is taxing and not all sluglings survive - weaker sluglings are not expected to make it and "disposed of". Because of this, the average mating will yield one or two slugman offspring (ie one per parent if all goes well).
Rarely, a clutch will yield two sluglings of superior quality with no clear winner. In those cases, they will both be metamorphosed. If they both survive, they are called "clutch siblings" and are the equivalent of twins.
This biology results in a complicated network of family ties. The slugman who provided the spermatozoa is considered the "father" and the slugman who carried the eggs the "mother" (slugmen who adopt genders will usually only perform one of these tasks. To genderless slugmen, there is no gendered connotations). Slugmen who are born of the same mating (but not the same clutch) are called "mate-siblings" and are considered almost as close as clutch-siblings. There are also couple-siblings (born of the same two parents but a different mating) and parent siblings (who share one parent). Cousin relationships are tangled indeed.
The key role the Teamaster plays in reproduction gives them tremendous power in slugman society. They are never the leader of the house, but are often a neutral party that can help arbitrate intra-house conflicts.
Teamasters constantly refine and experiment with their incubation protocols, trying to match bloodlines, ingredients and rituals to produce powerful slugmen offspring. The recipe and rituals that will successfully create a slugman that can wield magic is well understood, but this results in an individual who can only cast a few spells and is quite lazy - ie the average slugman. To generate the type of slugperson who will be more capable - the leadership type , or the adventurer - requires much experimentation. A bit of powdered dragon scale might result in a sorcerer instead of a wizard (to use 5e terms). A competent teamaster is key to a house's success.
If two slugmen breed and no Teamaster treats the eggs... you get slugs, nothing more. Slugmen who are considered abject failures may get to mate, but they won't get to breed - why ruin a future offspring with bad genes?
Slugmen have eyes on stalks/tentacles, and have 2 "chemical sensing" stalks they use to smell and taste - they dip these tentacles in their food before eating it. Their mouth contains a large powerful tongue studded with small teeth-like projections which they use to rasp at hard food. This mouth - the "eating mouth" is not used to speak as it is not connected to the lung. The fact that humans speak with their eating mouth is clear evidence of Slugmen superiority.
Although many slugmen have very refined tastes, they are omnivores that can eat almost anything organic - some slugmen dishes are simply unpalatable to humans.
Slugmen breathe through a large pore over their right shoulder - the pneumostome. This opening is also used to speak and is known as "the speaking mouth". Slugmen breath slower (2-3 breath per minutes) than humans but in larger volumes. To put your right foot forward in slugmen society is the correct one, so that the breathing pore faces towards your interlocutor. Putting your left foot forward when speaking is disrespectful as it faces the speaking mouth away from your interlocutor and puts the anus forward (which is near the left shoulder).
Slugmen have two independent eyes on tentacles. They do not see very well at long distance but have effectively 360 degree vision. A slugman who keeps both eyes on something is very intrigued by said things.
Slugmen have hands with 3 fingers and one thumb, and feet with 3 toes. Their fingers have a few hard knobs on them vaguely analogous to nails on human
Slugmen are mostly boneless, but have an internal vestigial shell that helps protect some of the organs. A secondary shell helps protect the six-lobed brain.
Slugmen prefer humid environment as very dry conditions can desiccate them if they don't drink enough water/tea.
Lastly, slugmen are resistant to poison in general (all those opium binges and capacity to eat almost anything) BUT they are vulnerable to alcohol. Primates - and humans in particular - are far more resistant to alcohol than most animals. How do you think an elephant - a 5 ton animal - can get drunk on eating fermented apples?! Because our capacity to drink is abnormal, and they don't have that. Treat each drink a slugman drinks as the equivalent of 8 drinks, at least.