Sorry, but imo, you're just being a prude there if you're "bothered hearing about people like that". If it's not your thing, it's not your thing, but if you're at the point of actually being bothered by someone else's active sex life, you should rethink the subject a bit imo.
Sex is fun. It's great. There's no reason to avoid it. And don't jump on HIV, yes, the risk increases the more you have it, but you have to be having sex with a person who has HIV (something that's rarely a case) and have the condom fail it's 98% protection or whatever. If you wanna live by those odds, buy a portable bridge and use it to cross the street to avoid ever being hit by a car. And then cry in agony when it goes onto the curb and still hits you.
Anyway where was I. The point is, I understand where you're coming from, personally. I myself am not big on jumping every chick I meet and casual sex is not the first thing on my list of priorities in life. I much prefer a solid relationship and someone I care about.
But a sexual friend is not an alien thing. It's not something to be feared. It's just the next step and we, as a society on the whole, are usually afraid of next steps. Are you telling me that you've never had a friend who you were somewhat intimate with? It's usually called a "best friend" but with a denotation "of an opposite gender" (sorry I guess this is a generalisation, nothing against gays/bisexual peeps, just talking from a heterosexual PoV here). You might feel comfortable doing 90% of the things couples do, might even spend more time together than a couple does. You might hug, cuddle while watching stuff, etc. Basically, all you're not doing is having sex.
So why not? I'm not saying every such relationship should do it, but it's not some "omagawd, stepping over the line" sort of thing when some do. That's basically what an open relationship is, a good friend you have sex with.
Sometimes it can be purely mechanical and some people do it just for the sake of sex itself, but hey, it's just a step up from wanking and I doubt you'll tell me you think that's wrong&evil. And I should remind you, the previous generation of prudes did refer to wanking as wrong&evil.
Anyway, it boils down to social evolution. A new generation will often take a step forward and if we're not a part of it, we'll likely feel the urge to think it's wrong. It's a built-in stopping mechanism and yes, it's purpose is to make us rethink what we're doing. So rethink it, don't just blurt out the moral outrage based on nothing but that built in feeling.