In the description of this perk it is said that men are easier/much easier to persuade. Does it have some kind of additional charisma equivalent? E.g. 1 level of this perk is like adding 2 charisma when speaking to men or does it have different mechanics?
It does not appear to have an impact on charisma, per se, but according to this site, there is a bonus to persuasion chance, which is broken down as follows:
Persuasion mechanics for this Perk are simple: Rank 1 Black Widow/Lady Killer gives a 10% easier chance to persuade, Rank 2 20%, and Rank 3 a whopping 30% increase to persuade in dialog.
I don't think it affects Charisma at all.
It seems that 'sometimes gives you additional dialogue options' is the extent of it (and the combat perks, but nothing to do with charisma).
Different authors of wikis and forums say, that this perk is implemented like the one with the same name from Fallout 3. So you will get extra 5% damage per level against male characters and you'll be able to unlock special dialogue options. (here you will find a description of the perk in Fallout 3)
After completing the game I also didn't find anything about special unlocks or loot when using black widow.