Thor....I think lizandtom did make a good point.
For me personally, it's not jealousy per se if I see my sweetie getting stupid over someone else. He's made comments/statements before about his preferences and what kinds of women he likes...and quite honestly, they are worlds away from what I am. I'm not petite, athletic, small titties, etc.
Not to say we haven't played with a few women that fit that description, but if he does get hugely excited over a playmate (particuarly if it is someone that fits his 'preference'), then it's more of a kick in the stomach kind of feeling, not so much a green eyed monster kind of feeling. Hopefully that makes sense.
So, if he does play it a little cooler...it puts me more at ease (even though I know that internally he's probably all "wooooohooooo!!!!!" and doing back flips
). It's hard to explain why that puts me at ease, but it does.
I don't really distinguish between the female of another couple or if our playmate is a SF. Unfortunately lust and other emotions are not rational and there is a potentiality for someone to get all goofy over a playmate regardless of their relationship status (and I think that goes both ways, male or female).
Just my .02.