You've always been physically close with your daughter, having play fights, tickling and goofing around. You have to be a bit more careful nowadays so your hands don't end up in the wrong places, but accidents happen... Like tonight. Your hand slipped when she was squirming around under your tickling fingers. You really didn't mean to, but suddenly your hand is between your daughter's legs. When you quickly pull your hand back, she tells you she liked it - she liked it, and she wants daddy to touch her there again.
With your wife in the house, you just CAN'T do things like that... but the feeling of your daughter's soft mound under your palm won't leave your mind, even though you know how absolutely wrong this all is. So when your daughter comes to you the next day and says she'll make sure to get mom out of the house for a while so you can explore more together... of course you agree. Even though you're family, even though it's the wrongest of wrongs, you want to feel that pretty pussy again - with your hands, yes. But with your aching dad-cock too.