Hagino is the original Ai.
Hagino is half human.where are they now? There are
Yui is Hagino and Mari's descendantMari and Hagino did this and that, leading to Mari getting pregnant. Yui could be as close a descendant as being their grandchild, since it's indicated that the Arume can become very old, which might also apply to their hybrid offspring.
The Arume actually don't need men for their species to surviveWhile it is a popular theory amongst some Arume in Tenshi no Bokura, it's shown in the original manga that the Arume are still a fully functioning species 1000 years later with no ill effects even though they still have no men. In that time, it seems they've either found a scientific fix or they've evolved one naturally.
The Blue Drop universe was one of the customers that lost the bid to have Gordon Freeman save it.
The Arume do need men for their species to survive, but not right now, or in the near futureWhatever genetic decay makes retrieving Arume men neccesary is ridiculously slow, so much so that they have thousands of years to decide; until then they'll just have a steadily-but-very-slowly increasing rate of genetic disease/problems and congenital birth defects, probably a few tenths or hundredths of a percent per decade, so a thousand years later it still isn't that noticable, and is easily mistaken for environmentally-caused problems; as the problem is so slow to develop, most Arume in the know don't see it as being worth it to change things. Being that those in the know are major leaders, they know that if they do change things they'll face major opposition, because its simply too long-term a problem for anybody to seriously worry about compared to the inherent desire to maintain the status quo. Eventually- far into the future when peace exists between humans and Arume, thousands of years after even the one-thousand-years-later series, the incidence rate of defects and problems will climb high enough that the populace will get restless and angry and some enterprising Arume politician will bring out the "men" solution and pretend its a new revolutionary idea- it having been kept secret just so they could reasonably try and claim that- and things will move naturally from there to a better ending.