Issy I wonder why you said you would love to sleep with your new friend? Perhaps you would say that it was the kindest way of saying no thanks, whereas another interpretation might be that you want to keep open the possibility, especially as another meeting is planned? When I mentioned downthread about secrecy, this also refers to the content of the interactions. Does your H know then, that this man wants to sleep with you and that you have told him you feel the same, but won't?
I agree profoundly with you about the very different nature of friendships and my sense of this thread is that amongst the sarcasm (and especially in MotherInferior's case, the wit that also made me laugh out loud) there is huge misunderstanding about the points some of us are making. Now some posters respond by heaping scorn, anger and sarcasm and others want a reasoned debate.
The fact is that infidelity does occur between friends. This doesn't make the friends bad people and it doesn't mean that people with safe boundaries are hysterical monogamists obsessing about their partner's thoughts, actions and friendships. It doesn't mean that people can only have friends of the same gender and it certainly doesn't mean that couples must go everywhere together in tandem.
It seems to me that these points are wilfully ignored by posters who would prefer a stereotype of a jealous obsessive who polices the friendships in her relationship; her own friendships and those of her partner.
This is about safe boundaries, of the kind that you observe in your first example, Issy and most closely represent the friendships I have with my own male friends.
Years ago, I can also remember friendships that more closely resemble your second example and saying much the same as you - and would be disingenuous if I said that this was about being kind to the interested friend - rather, it represented an ego boost and a delicious buzz. This was however at someone's expense - my H's and the friend's partner, both of whom would have been less than delighted that their spouses had admitted strong sexual desire.
Pretending that friendships are always non-threatening to fidelity seems wholly disingenuous and ignores the facts and the truth. They don't have to be threatening, but in a way, the person who refuses to acknowledge the possibility is the very person most vulnerable to hurting their partner, even if they never had any intention of crossing that line when the friendship started.