Thought I'd post this recent exchange because it's such a common question, and SO many couples are in this situation. I've been reading Spanking the Male Mind in my spare time. I haven't yet presented it to my wife. I have approached her about wanting this before but she basically just chalks it up to nothing and nothing happens.
Masochism is quite common in both sexes. What percentage of women like to be spanked? For one thing, buttock-battering can vibrate the pudendal nerve, which can lead to sexual arousal. There are multitudinous other physiological ways in which it can be sexually… abusive, but I won't list them all here.