This reminds me a bit of the conversation we had at the group where we talked about casual sex and poly. We ended up agreeing that people have different views on what we termed "recreational sex". Some people view sex as an activity (rather like dancing? .. I'm trying to think of an analogy that is physical and pleasurable that one does with friends as well as partners, here). Other people view sex as an emotionally intimate act that is something to be shared only with a select few: those who meet a certain level of emotional intimacy. Quite a few of us fall somewhere in between on this spectrum.
I have no idea why some people are wired to enjoy recreational sex and others aren't, but I suspect it's a complicated combination of experience, culture and possibly physical/chemical elements (similar to the reason some people are highly sexual and others don't have much interest in sex at all). My personal view is that it's all good, so long as people communicate their preferences to each other.
I'm one of those people who falls into the category of "someone who can be sexual with friends and not feel weird about it afterwards". I find sex deepens my bonds of friendship OR romantic love with those I engage with. However, I don't feel a need to be "in love" with everyone I have sex with. I view sex primarily as energy which is healing, healthy and pleasurable. I do not want to share it with everyone (because I'm not attracted to everyone) but I don't want to limit sharing it with only those who are my "partners".
I'm careful to ask people how they feel about sex before I engage in it with them. I'd rather keep a friendship than cause hurt or confusion by adding sex to the mix if it's not compatible with their beliefs or values. I value my own sexuality and the sexuality of others. I guess the closest I can come to describing how I feel about sharing sex is that it's almost a spiritual experience
for me, though of course it's also physically pleasurable and is one of my favourite ways to express intimacy with my partners. Sex is sacred, but for me that doesn't mean it should be kept for only a few people. I feel a special connection with everyone I've had sex with, whether they are friends or life-partners. I just don't equate this special feeling with romantic love. It's more like a sacred love and connection (and this can coexist with romantic love, but it doesn't have to).. does that make sense? Surely I'm not the only person who feels this way!
I agree with Redpepper about the definition of "sex positive"; this has less to do with who you fuck and more to do with how you value your sexuality and others' sexuality. You can be sex positive and never have actual sex with anyone at all!
Oh, I also wanted to add that I don't have any trouble separating out friends from lovers/partners in my own mind. I just don't use sex as the criteria to distinguish between them.
Anyway, that's sort of rambly, but hopefully it helps add another perspective!