2., 3., 4. Black Widow, Lady Killer, & Child At Heart
What Black Widow/Lady Killer does: You do +10% damage against opponents of the opposite sex. You'll sometimes have access to unique dialogue options when dealing with the opposite sex.
What Child At Heart does: This perk greatly improves your interactions with children, usually in the form of unique dialogue choices.
For the people who decide to play Fallout 3 as a man, understand that the Lady Killer has a great flaw: you will not face a lot of enemies who are women. Besides some female raiders and soldiers, that's about it. But beyond that, there is no Super Mutant lady, robot nurse, or mother Mole Rat. The perk's 10% damage to a female then loses its value.
The Black Widow will provide a better experience since the Wasteland is filled with bad men needing to get their heads blown. Still, this perk is not necessary thanks to all the loot you will find that keeps your inventory stacked with weapons, ammo, and armor, so that 10% don't mean squat after that.
Child At Heart is a weak perk because you won't meet many kids in the Wasteland. In Megaton, the first town you visit, there is basically two kids who you talk to in that entire town. One place that is the exception would be Little Lamplight, where many kids are living, but more than likely you won't even visit there until a long ways in the game. Adults will be most of the NPCs you see in Fallout 3.
Even though there are benefits behind these perks, where a few characters will give you items or perform special services for you, the fact is you have no idea who those people might be. The game doesn't tell you, so all you can do is hope that the next person you talk to is one of few dozen who might share you a secret or do something nice.
Had the game provided some information or clues that imply to people, places, or quests which the Lady Killer, Black Widow, and Child At Heart perks would be useful, then these would have far more value and be less of a headache.