Although it's more /a/ related but I don't think I can get answer there:
In certain episodes of K-on, Mio said "papa" and "mama" but after she said that she looked embarrssed and soon correct her word to "otousan" and "okasan", why? I do know the word "papa" somehow relate to "enjou kousai" and "mama" might has something to do with prositution but isn't calling your parents "papa" and "mama" quite normal these days.