Never? Bzzzt, wrong answer. And surprise! - it's got nothing to do with religion.
When sleeping around is very indiscriminate, with many acts and partners, it doesn't just spread disease. The diseases themselves get a chance to evolve rapidly. Diseases that had been mild because they had to permit their victim to live long enough to infect someone else can survive in more virulent form when they have more opportunities to be transmitted. Next thing you know we have ever more dangerous and more prevalent diseases, venereal or otherwise.
Now let's suppose that previously exotic practices become commonplace. Suddenly herpes viruses can find a couple more warm moist places to proliferate, on the genitals and anus. The microorganisms of the colon are not compatible with those of the vagina, so associated health conditions start to affect more women. All of a sudden blood from partners can mix on a previously unknown scale, leading to the emergence, prevalence, and virulence of AIDS. All the extra antibiotic usage helps to develop more strains of resistant bacteria, both for the intended diseases themselves and any others that might have been exposed.
So in fact it turns out that someone else's sexual practices can have impact on your health even if they don't sleep with you, and if you're indiscriminate you're part of the problem. Especially to the millions who must use needles for their health or profession, which has resulted in large numbers of straights getting AIDS despite avoiding risky behaviors.
(So what do we send them to combat it? Condoms. Go figure. People who would sneer at fatties or drunks think that it's unreasonable to miss a chance to get laid.)
So remember - when you hop in the sack with a new partner, you're not just sleeping with everyone else they've ever slept with. You're sleeping with me too....