There are various theories as to why so many temples are decorated with such unusal carvings. The most popular is that the godess of lightning is personified as a shy young virgin and would therefore not want to go anywhere near buildings with such scenes on them. personally I find this explanation unlikely since it does not explain why they don't include erotic carvings on secular buildings. The real explanation is probably more to do with the tantric aspects of worship, borrowed from Tibetan Buddhism and perhaps even with the Newari sense of humour - since many of the scenes are extremely amusing. My favourite is a rather worn carving on a temple in Bhaktapur which shows two elephants having sex in the missionary position...