Masturbation
What's so shameful about wanking? Nothing, actually.
His and hers
Masturbation is the act of stimulating the sexual organs for pleasure. It isn't compulsory, nor is it a criminal offence (unless you're doing it through your pockets in the dinner queue or something). A lot of people enjoy masturbating - male and female, regardless of whether they're in a relationship.
The trouble is very few people ever admit to cracking one off in private, and as a result a lot of guilt, anxiety, myth and misunderstanding surrounds the practice.