How do I explain to my girlfriends that I'm not going to have any formal bridesmaids?

I like the man of honor idea. Hopefully your girlfriends will be accepting. I suggest you have them rent dresses or YOU buy them dresses. I don't think it's fair to ever make a woman you request in your wedding buy her own dress, and in this case, they aren't even in it! You could possibly have them rent dresses, or just get nice dresses they could all wear again in the same color.