If I recall, Ian Thorne's Godzilla book stated the original Godzilla was male and the second ("Gigantis") was female. This had me fooled for a while as a kid, taking this as fact, but was obviously false. I've pretty much always believed Godzilla to be male, as KPA pointed out, there's nothing official that states otherwise.
Questioning where Minilla came from is no evidence at all of Godzilla's gender; Just look at Baby from MG II, [i]he[/h] didn't come directly from Godzilla.
And while named "Godzilla", the American 'Zilla isn't the actual Japanese one, so you can't really apply his asexual trait to the real Toho Godzilla.