I wouldn't even consider it that gory to be honest. I think Doom is more gory then Duke though some of the cutscenes make me giggle like when he kicks the eyeball through the goalpost.

The game's pretty tame compared to what's out there right now. Also, there's barely any nudity to speak of and what's actually shown is barely distinguishable. I personally played this for the first time when I was pre-teen and I think I turned out pretty normal so my guess is there's little chance of you turning into a menace to society just because you've played some DN3D.
It's also a lot of fun.
You've obviously never met the right pair. :-)

here are the game pages for thew duke games at 3dr/apogee ltd
duke 1 the page now shows a teen rating
http://www.3drealms.com/duke1/index.html
duke 2 rated E now
http://www.3drealms.com/duke2/index.html
Duke Nukem 3D: Atomic Edition rated m
http://www.3drealms.com/duke3d/index.html
Duke Nukem: Manhattan Project rated m
http://www.3drealms.com/dukemp/index.html
Duke Nukem 3D for Xbox LIVE Arcade rated m
http://www.3drealms.com/dukearcade/index.html
the game was very controversial when it came out seeing that you go through strip clubs and adult video stores.

Of course you should get this game, it's a classic. Also, there's a parental switch in case Mom doesn't like the gore and swearing.
Although to be honest, the swearing is probably stuff you have already heard and the tits are rare and pixelated, nothing to get excited about. The mature content in this game is actually pretty laughable by today's standards.