There is no mens group. The suggestion is put up regularly, though it seems like women suggest it more than men. I've seen mens sections on Christian forums. They are rarely used.
Far as why the teens forum isn't isolated only for teens i think is because it gives adults a chance to use their experience and wisdom to help the younger generation. There's nothing really that is something that only another teen needs to read, unlike the womens forums which may have topics that some women would be sensitive about having men involved.
Also, if we're going to take this attitude of isolating certain forums then The Singles should only allow 18+ Single people. Indian forums should only allow east Indians, etc... and once all of these distinctions were isolated from the rest of the forums it would cut down on the available options to post in.