Who it's for, and when
Who it's for: all men, single or in a couple, who'd like to move beyond the quickie. Men who want to learn to delay ejaculation will find it their best training ground. And curious partners: taking things in hand is also a way of learning what gives a man pleasure.
When: whenever you have time to spare. For pleasure, to unwind, to release tension, or when your partner is tired or doesn't share your appetite. In a couple, touching yourself doesn't replace making love together, it's a valuable extra (plenty of men who have sex regularly masturbate too).
Guilt-free: many men carry a lingering sense of embarrassment or guilt from their upbringing. You're not the only one, and there's nothing bad about making yourself feel good: masturbation is part of sexuality, solo or in a couple.
Precautions: on the testicles and perineum, keep the pressure measured, never forcing. Pain that persists, ejaculation that always comes too fast and weighs on you, or erection problems every time you have sex are worth taking to a doctor or a sex therapist. The same goes for any prostate trouble.