It's been awhile since anyone posted on this thread but I came across during an internet search and while I agree with many of the replies, I wanted to chime in.
I haven't lived in Japan for many years so I can't accurately comment on the current speech trends of youth but I still listen to some music and watch tv now and then as well as speak to other Japanese and visit once in awhile. I am half Japanese and learned most of my language skills while living there as a kid but it was as a second language, though picked up much more like a native than someone trying to acquire second language skills from a textbook. I grew up a tomboy and tend to speak more casual/rough in both English and Japanese but as I got older, my Japanese has become more polite. I did not learn to speak from men (unless you count pop culture) so I discount the theory of gender nuances acquired by surroundings. I learned from my mom, female relatives, female friends, etc. Therefore, I guess that rougher speech may be common amongst young females but as they grow and have to fit into Japanese social norms, their speech become womanized. I could be wrong, just my guess. I notice in English as well, very few people call their relatives by their first names, but I do (except for direct descendant elders, such as mother/father, grandfather/grandmother, etc) and I think that's an example of polite/casual. I did the same in Japan until I realized it's a bit weird because they're more polite than America so I think once I hit my 20s I started using おじさん to my uncle but with his first name in front.
When I left, I had just started to pick up on the few girls who dared to say 俺 or 僕 and I thought it was really weird. "Why would you want to say that?", I thought. (A contrasting rarity, one Japanese girl who came onto a bus I was riding spoke the dirtiest, filthiest English I've ever heard as she bossed around her American friend. I have no idea if she knew the tone of what she was saying. I can just image her using masculine speech in Japanese though.) I would also never say あたし. That is so prissy! That's like girly girls and old ladies on tv programs from Tokyo. I would get upset at myself if I caught myself ending sentences with わ because that was too girly in my eyes as well. I doubt I've ever said じゃん--I may not always use です／ます but I'm not that heavy into contractions (how every unJapanese of me), perfectly fine to go all the way to じゃない. I often ended sentences the way many of the "masculine" speech is described here so I would argue it's said by both men and women, but more so for men, such as ぜ (such a great one to use but yes, not many women use it), よ and だ (trust me, many women use these). Why is ぜ so great? Because it's exciting! Let's do something! It pumps you up. I don't I said ぞ much, if at all. That's kind of like a bossy dude. I don't find it that motivating. Unless you're at the start of a race or something, 行くぞ！
I've read somewhere that young men tend to say 僕 and I disagree. I've noticed older men say it as well so I don't think it's limited to age. My mom's boyfriend says it (70s), my uncles says it (60s), the businessmen at Toyota when I worked there said it (50s), etc.
Any speech that is definitely feminine used by men, such as あたし, I would definitely peg them as being gay, even if they denied it. That's just how off-putting the Japanese language is. However, I wouldn't consider a female gay if she used 僕 or 俺 but I would seriously question her speech choices and if she never uses 私 then I would find it too off putting to continue conversations with her.
I would also argue the notion of using the word "dialect" when referring to Tokyo. Tokyo is standard Japanese. There is no dialect. There might be language trends but not dialect. I've met several people from Tokyo who lament that they do not have dialect. I never thought of dialect so warm-heartedly. I thought of it more as culture to a slight nuisance. I mean, when you get engulfed in a region, you might not even realize what's 方言 and what's 標準語 until you start speaking to someone in a different region and they can't understand you or that you're not employable because you don't speak polite Tokyo Japanese (yeah so maybe I had a few people in Tokyo laugh at me when I visited in junior high with my いなかくさい talk). 大阪弁 baffled me as a kid but later I realized some common words are from there, it's that tv has made them common because that's probably where I absorbed them from. So to the theory of where words come from, I can't say but more and more Japanese move away from their region so it could spread like that but I would guess that Japanese would try to assimilate to their new region and not use their 方言 to others. Sorry for the long post.