If you mean the general sex-obsessed culture at university then it's not just freshers week, although I admit there's certain reputations for people going "mad" in freshers week in particular. However, in my experience and knowledge, most people I know had way more sex in general throughout the year rather than just freshers week.
As for the question, my thoughts are that I don't think sex is promoted that much. It's talked about, joked about, implicitly implied that it's heightened at uni and that's what everyone does etc, but I'm not sure I'd use the word promoted? Who exactly is meant to be promoting it? If people want to have flings, sex buddies and one night stands at university, fine by me. No babies and no stds flying around and I'm good.