I've chalked it up to it initially being standardized injections at birth like vaccines that modify gametes of species and make it possible for interbreeding that also results in fertile offspring. This was done to smooth over inter-species relationships and create a piece of common ground so that aliens don't feel that alien and that less conflicts break out due to people believing that they are too different to coexist peacefully. One of the greatest unifying forces between two tribes is for the different heirs to have children, or so they say. The specifically programmed nanomachines that can change people was a coverup to hide the first little buggers from being discovered and to continue funding their endeavors. A haystack with a needle in it that pays for more needles in more haystacks, so to speak.
Eventually the industry became too expensive and they needed a way to automate and cheapify the process, resulting in the things becoming airborne and capable of automatically reproducing themselves. This has resulted in anything that was in an atmosphere with or near U.G.C territory citizens becoming capable of inter-species reproduction. The rush was a media and culturally-promoted way for the airborne nanites to propagate throughout the galaxy, further spreading and blurring the lines between species as all become compatible with one another. Eventually creating peace between all peoples as the tainted space that is U.G.C territory spreads
The true goal of all this? Total galactic control. Once all species have made peace with each other they'll unite everyone under one rule, for better or worse. Perhaps it is to face a threat greater than anything inside the galaxy can individually face, or maybe it is to enslave everyone so that those at the top can simply become more powerful as time goes on. And with advancements in the medical industry that counteracts the aging process they'll eventually be immortal, forever in power, forever their rule absolute.
Or maybe I'm just crazy. Who knows?! HAHAHahahahahahahahahahahahahahaha... *coughs*
