These bug me more than they have any reason to. Hell, I had some truck with "matching gangs" myself in my erstwhile "Keith Randall" stories, where two Hollywood heartthrobs met two fans who hung around together and so on. I remember it making sense along the lines of, "Oo, I could gossip with my friend and we'd know everything because everyone would always be together!" This isn't a healthy attitude, but it's not in and of itself peevish on this issue. It's these particular boys that make it weird (and no, not because I think they ought to have a band of matching Slytherin male pranksters, with Lily only appearing in order to provide one of them with a son).
I thought at first that it's because the boys are so different from one another that it's highly unlikely that they'd find an exact mix of girls to suit them who happened to be friends. This is pretty unlikely. The boys are friends initially because they all live together and, by all appearances, James's strong personality binds them into a family. Is it likely that the same mix would end up in another dormitory, rather than spread among years and houses? Yeah... that's still pretty problematic. But that wasn't it.
I think in the end, it's because the Marauders have such a tight bond, and seem to look to one another first when it comes to emotional needs. If there were a gang of girls with Fem!James, Fem!Sirius, Fem!Remus, and Fem!Peter, then the only possible purpose they serve is sexual, because if the person you're looking for is James, then Fem!James becomes superfluous. And if their only purpose is sexual, why bother mirroring the personalities?
I think Marauder-fics work better if the boys are paired with people who fit in their lives differently from their other friends. Not necessarily in some opposite way, and not necessarily with wildly different sorts of interactions--people you like are people you like, after all, and there should be some kind of continuum between one's friends and one's lovers. And it's not really that much of a different proposition than the idea that among the friends themselves, Remus is different from Peter is different from Sirius is different from James. (Sirius and James may be outwardly similar, but I presume they complement one another in some way, otherwise one becomes kind of repetitive of the other, which would be grating after awhile!) So any Significant Others who come in would be distinctly jarring if they repeated characteristics.
As I said, this isn't to say that there can't be similarities. What one of them chooses in a mate may have a lot to do with a slightly different combination of factors that he loves in his friends, as well as her own unique contributions. I've noticed in most of the successful marriages I've seen, the spouse is perfectly able to "fit in" with friends and family, and carry on conversations on a similar wavelength on frequent occasions... but the person is not the same, or "equal to" someone else, because that person isn't replacing someone else.
I just think that the ships for any group of friends work better when the new person brought in brings something to the table that wasn't there before. Lily seems to bring a kind of clear-eyed sense of justice and responsibility into the group. Tonks--when not depressed--brings a sense of fun that doesn't include the levels of cruelty that James and Sirius indulged in. And so on.
And that's it. I'm going home now. :)