I'm pretty certain it's a rumour, I tested both yesterday (and why do I get the feeling that it was for the same reason you need to know );
With a female Plusle and a male Minun, I was getting baby Plusle's, and with two Minun's, you got it, baby Minun's. So unless getting it involves having certain species as certain genders then I don't see how it could work. Or my Ruby could be messed up:-/ But somehow I don't think that's the reason (the messed up option).