Dad, what's the difference between 'in theory' and 'in practice'?

The dad thinks about how to best explain this to his son. He goes into the living room and asks his mother-in-law:

'Granny, tell me, what would you do if a tall strong man gave you $5,000 to sleep with you?'

'Well, son, at my age one can't be picky, and besides some extra cash would come in handy.'

After that, the dad goes into the kitchen and asks his wife:

'Honey, tell me, what would you do if a tall strong man gave you $5,000 to sleep with you?'

'Well, you know I love you darling, but we need the money, and besides a vacation would be welcome, so I guess I'd sacrifice myself...'

Next, the dad walks into his daughter's bedroom and asks:

'Katie, tell me, what would you do if a tall strong man gave you $5,000 to sleep with you?'

'You know dad, I'm 18 now and I'd like to try it, especially if I could get that much money for it.'

The dad then turns towards his son who had been following him around and says:

'See, son, in theory we have $15,000, but in practice we live with three skanky whores!'