>>16976492
It predates animals by a lot, like a billion years. Depending on how you define sexual reproduction it evolved either about 2 billion years ago in early eukaryotes as it's more structured form or since time immemorial trough more ad hoc gene transfers that things like bacteria still use today.

>I just wonder if alien life would also have male and female sexes or if alien life would have any other kind of system going on
Sexual reproduction offers many benefits over the competition and it's still the dominant way of reproduction for basically all multi cellular species especially the more active and competitive animal types. It's reasonable to assume that this would hold true with aliens. 2 sexes is also the most stable form of sexual reproduction so it would also make sense that aliens would have what we would describe as male and female though their roles could differ (for instance it could be that the one that makes the eggs would not carry them like how seahorses do it). That is not to say that a hypothetical better reproduction method isn't possible, who knows with sample size of 1 planet.

There's of course many alternatives it would be quite reasonable for aliens to be cloning species trough some kind of budding process especially if they were plantoid or fungoid and hermaphroditic species would also be quite reasonable like your snail types, sex changing is also pretty common in nature. The sea horse style mixing of the roles could be very likely too.
As a personal favorite I find the symbiotic/parasitic 3rd sex by Pierson's Puppeteers from Nivens works quite interesting and plausible

If you are writing then you should just write something that is interesting, realism < fun

>>16976492
Meiosis and syngamy are extremely complex and likely evolved only once, probably in the Last Eukaryotic Common Ancestor (LECA) 1.5–2 billion years ago. Animals, plants, fungi, and protists all use variations of this same toolkit.

Early sexual organisms were likely isogametic, but over time, evolution created a kind of optimization game: some cells benefited from becoming tiny and mobile so they could be produced in huge numbers, while others benefited from becoming larger and nutrient-rich to improve offspring survival. Because this tradeoff is so fundamental quantity versus investment I think different lineages likely converged on similar systems independently. In other words, sex itself may have one deep origin, but male/female specialization is probably an evolutionary solution rediscovered over and over again.

Given my bias that anisogamy (male/female) is an optimization game of resource distribution (numbers vs. investment), I would bet that complex, multicellular alien life would also convergently evolve something analogous to male and female.
If alien life is complex, it will face the same trade-off - do you make many cheap, mobile gametes, or a few large, resource-rich ones? The stable solution seems to always be both. They might have three sexes (e.g., a "large resource" sex, a "small mobile" sex, and a "medium" transport sex), or fluid hermaphroditism, but the basic economic principle of anisogamy should hold up. Life that stays simple or lives in extremely resource-abundant environments might stay isogametic, tho

>>16976568
>>16976591
These answers are really good and super interesting, thanks anons!
I wasn't planning on writing anything, just kind of speculating on what alien life would be like, but I do also agree that fun is more important than realism in fiction unless there's an important political message being told, but that doesn't matter right now

masturbation is not sexual reproduction

>>16976597
alien life will be alien we might not have answers by looking at our evolution history

for example we have 2 sexes because that was the most convient way since a few hundred million years ago convergence allowed it, but there could be something like prime immotile from the hamilton commonwealth series, a huge hive mind species that makes motile copies of themselves like soldiers of an ant colony, they only "grow" by synthesizing information, alien life could be very alien

>>16976858
It's more likely that alien life is similar to life we know of than for it to be different, since at least we know for sure our method works

>>16976492
when they were created