Sekkusu

'Sex' is a natural for being written in English as 'Sex' rather than in katakana as セックス sekkusu:

1. The English, at three letters, is shorter than katakana, which writes it as four letters. It is easy to spell and has a bolder look.
2. The English has better standout against a Japanese text as background. It also looks more sophisticated than katakana.
3. As a foreign word, an attempt is usually made to pronounce it as 'sex'. Pronouncing it as 'sekkusu' sounds forced and unsophisticated.

The following example from an advertisement for a monthly magazine is a good illustration.

Sennyū: Ichiban atsui yoru no machi Nishi Kawaguchi, Kabuki-chō, Roppongi, Nagoya, Oosaka Fukuro-toji Chō sekkusu supotto Infiltration: Hottest streets of the night Nishi-Kawaguchi, Kabuki-cho, Roppongi, Nagoya, Osaka Sealed section Super sex spots

A Google search in August 2003 revealed the following distribution on the Internet: