I write mostly male characters and my characters swear. Not constantly, but when angry, during sex, and when excited, the f-bomb drops. Yep!
I'm sorry, but men cuss...flat out cuss. They may not do it in front of an audience, but I'll tell ya what! I have worked around men my whole life (chose a male dominant profession) and I don't care how proper they are in the office/garage/kitchen...get them in a room with the 'buds?' Watch out! Hahahhaahahahha! Hell, work with them long enough like I have, and there's no censorship at all! Some of the funniest things I have ever heard were men chatting it up amongst themselves and the language flies! Pays to be the boss with an open ceiling on your office!

BWahahaahahha! Too F'n funny!
Seriously though, it depends. I agree with many others for the most part, but.... Here's the thing with character stereotypes. If a character is a mellow, withdrawn computer geek, one might assume they may be more 'proper' with their language, but who's to say what that person might be like with a drink or two, in bed, blah, blah, blah. Follow me? If you want realism...really think about your characters persona, environments, associated characters. Really research men in their own environment, women too. Hit a club, a bar, a convention, a concert. Be a fly on the wall so to say. Oh...and get a bunch of women together? Holy Crap! Sometimes I think they are WORSE!
