An old flame
    An old flame is a person that somebody has had an emotional, usually passionate, relationship with, who is still looked on fondly and with affection.

As the actress said to the bishop
    (UK) This idiom is used to highlight a sexual reference, deliberate or accidental.

Facts of life
    When someone is taught the facts of life, they learn about sex and reproduction.

In the club
    (UK) If a woman's in the club, she's pregnant. 'In the pudding club' is an alternative form.

In the family way
    If a woman is in the family way, she is pregnant.

On the game
    (UK) A person who is on the game works as a prostitute.

On the rebound
    If someone is on the rebound, their relationship has recently ended and they are emotionally unstable.

Play the field
    Someone who plays the field has sexual relationships with many people.

Shotgun marriage
    A shotgun marriage, or shotgun wedding, is one that is forced because of pregnancy. It is also used idiomatically for a compromise, agreement or arrangement that is forced upon groups or people by necessity.

Up the duff
    (UK) If a woman is up the duff, she's pregnant.

Up the stick
    (UK) If a woman is up the stick, she's pregnant.

Virgin territory
    If something is virgin territory, it hasn't been explored before.

X-rated
    If something is x-rated, it is not suitable for children.