that is an interesting idea- that between Reshiram and Zekrom there is the masculine in the feminine and vice versa- like a Taijitu
To me, Ho-Oh is like the [female] I ching element of Fire and sort of carries feminine traits in the notion of birth/rebirth- through parthenogenesis perhaps, something only usually achieved by female or hermaphroditic species.
Lugia is like the [male] I Ching element Water.
I think it is a serendipitous part of their design, if you look at Zekrom, you could suggest that there is something evokocative of a vulva or a womb in its design between it's legs. So it is a very interesting point that there they can both be seen as hermaphroditic and encompassing themselves and each other- which calls out the inseperability of these forces, though they can identified they cannot be truly parted- if you now what I mean; no sky without Earth, no light without darkness, no cold without heat.
Wow, I feel I might have licked one too many palpitoad backs haha