If you had been born into that world, I'm pretty sure you would have grown up accustomed to it.
I don't know what you're on about bro, but I can't imagine wanking in a world full of men. Where would I find material for masturbating if there's no traces of women?
You'd then probably be gay, so no problem finding "material" Just like many mammalian animals when they are kept in an environment from birth where they interact with only one sex. It's one of the reasons farm animals are neutered even if they are permanently penned by gender, to neutralize the instinct to mate.
Therein is the answer to the first issue as well. Hiding your genitals and pleasuring only in private has a couple of reasons behind it but one of them is to indicate you're not "available" right now. And culturally, since you'd be gay in that world, that would still be relevant.
Not everyone has to be gay, since humans have this annoying thing called "free will" that interferes with our instincts. But as long as at least some gays come about because of the environmental factors (and we do see that kind of behavior amongst some of the girls), then that social pressure to keep it private will still exist.