“In combat, you do +10% damage against female opponents. Outside of combat, you'll sometimes have access to unique dialogue options when dealing with the opposite sex.”— In-game description
In combat, the player character does +10% damage against opponents of the opposite sex. Outside of combat, one sometimes has access to unique dialogue options when dealing with the opposite sex, similar to the Sex Appealtrait. In Fallout: New Vegas, it is complemented by the Confirmed Bachelor perk.
In The Pitt, Vikia will prove to be immune to the perk and will mock women who are manipulated by it. She goes on to say that she wouldn't sleep with the player character, no matter how impressive he is.
One can get Sweetie to hand over over the key to St. James' room in The Coyotes sidequest.
One can invoke some additional dialogue between the Courier and The Lonesome Drifter; he informs the Courier that he lived in Montana and his father left before he was born. The player character has the choice of worriedly asking "Montana? Oh god, you wouldn't happen to be seventeen would you?" to which he replies in the negative, and the player character responds by attempting to change the subject.
“Come into my parlor," said the Black Widow. You're charming... and dangerous to those of the opposite sex. Though the gender of the Commonwealth's population is about evenly split, this perk won't affect those sporting a similar undercarriage to you. Don't forget the Intimidation perk; both of these perks in unison allows almost any situation to be handled your way, without (or with) violence.”— Fallout 4 Vault Dweller's Survival Guide description
Just as before, Lady Killer offers bonus damage against women. However, it does not confer any additional speech options due to a refurbishment of the dialogue system, and instead makes them easier to persuade in dialogue or be pacified via the Intimidation perk.