To offer a counter argument as to why it's okay for girls but not for boys - it's about social norms.
There is no way that any woman would be negatively judged in the job market for having pierced ears, because it is such a normal thing. In fact, well chosen earrings can even be a signifier for dressing well. Because it's so common, it's unlikely that a girl would seriously regret her decision when she got older, even if it was just a whim that she didn't care much about later on. So aside from the risk of infection, which is minimal if the piercing is cared for properly, the long term risk of it affecting a girl negatively is very low. Additionally, many girls have their ears pierced young, so there is a danger of really being left out of a typical bonding ritual that girls tend to do at around that age. Actual disadvantage from waiting.
On the other hand, boys' and mens' ear piercing is still relatively niche. Yes, some celebrities have their ears pierced, but most men you come across in life don't. It does give across a specific image, and this might reflect negatively on a man searching for a job. Even if holes are let close, you can usually see. I think that men are probably more likely to regret ear piercings, too, because of the fact it's not so common and does give across a specific image. So I'd say that there is a medium long term risk here. Secondly, there's much less of a disadvantage to waiting until he is, say, 16, and more able to make a decision for himself. As earrings aren't as common for boys, he won't be missing out on anything. The earring might buy him some extra "cool points", but that's not the same thing.
I'm a feminist and I would normally say absolutely what you allow for one gender you must allow for the other, but it's not as simple as that in this case, because the expectations and therefore the risks and benefits are not quite the same. A lobe piercing on a boy is similar to a non-ear piercing on either gender in terms of how it's likely to affect the person later on in life.