It's a silly question, but some people take it seriously. A few quick observations:
i) Gen 6:1-4 doesn't say the "sons of god[s]" were sky people. It doesn't say they came down from the sky.
ii) The syntax is ambiguous. It doesn't say the Nephilim were the offspring of union between the "sons of god[s]" and human females.
iii) Even if you think aliens exist and made contact with humans, there's no reason to think they'd find human females sexually attractive–any more than we find other species sexually attractive. Isn't it pretty egotistical to imagine that ETs lust for human females?
iv) And if it's aliens, why just male aliens mating with human females? What about female aliens mating with human males?
v) Moreover, it's wildly improbable to think that aliens, who (ex hypothesi) developed independently on a planet with a different atmosphere, different ecosystem, would be anatomically compatible with humans–much less genetically compatible to the point of interspecies reproduction, resulting in hybrid offspring.
I suppose a committed ufologist could salvage that thesis by appealing to genetic engineering. That, however, strays far from the text.
vi) Isn't the alien abduction trope about humans going up rather that aliens coming down? That humans are transported to a spacecraft to undergo experimentation?
vii) To me, the text invites a far more mundane explanation. Isn't this a familiar scenario? Raiding parties to abduct women from a neighboring tribe or village. That happens in lots of primitive cultures. An invading army where officers have the pick of the women. Sex-starved sailors who discover the Polynesian islands and help themselves to the bounty, including–or especially–native women.
If it weren't for one or two enigmatic designations (nephilim; "sons of god[s]), surely we wouldn't take it any other way.