Talking about sex is Sin? That sound suspiciously like repression to me.
If thinking, wanting, and desiring sex is sinful, I think you really need to look back on times like the Victorian Era.
In the Victorian Era in Britain, there was immense social pressure for sex to be completely restricted to marriage. Nevertheless, it was one of the most disastrous age for sexually transmitted diseases. A lot of it went on behind closed doors. Women were treated foully for their actions, but men were rarely held accountable. Rape was considered the womans fault for not being 'vigilant' or 'modest' enough (Sound familiar to any religions today?)
If you believe sex is for within marriage, fine. But not everyone has a peachy life and can achieve, or even want, based on their religion or lack thereof, a successful marriage.
Rape is considered a sin. Suppose a woman is forced to submit to her abusive husband. Now suppose a couple who really love each other have sex, but not until they have secure jobs and know that any children will have adequate support. Suppose the latter couple were not married.
Which is more sinful?