Jamie
I agree with you about no "type" of friendship being immune from an affair.
I think they are less of an unknown element though, iyswim.
e.g. DH is part of a large group of old Uni friends. Unsurprisingly there is lots of sexual history within the group - people who had longish term relationships, people who lost their virginity to each other, snogs, shags, "will they, won't they?"s etc. etc. My Uni friends are similar, but not nearly so tight as a massive social group so many years on.
But these people are now friends - they are not each others exes or old flames or whatever. They are friends, the history is history. It's part of the story, but it isn't in the present. It's easy to see that when you meet them. Occasionally someone will bring up an old sexual connection between two of them that the newbies (such as I) don't know about, and we'll all guffaw because it seems so incongruous now.
But equally there can be longstanding relationships, even ones cast officially as "best friends" where the subtext of the story is that the two are really "made for each other" or will get it together eventually, or there's some kind of thwarted true love narrative. And that TBH I would not be happy with, although you'd know early enough not to stick with someone who had some kind of "true love" thing going on with someone they pretended was just a friend.
And of course old friends could suddenly find themselves falling in love, but I do think in a situation of true friends that that likelihood is significantly decreased by the existence of a wife or husband that has become friends with the old friend.
Some of DH's women friends are some of my best friends now. They are all people I like enormously. I have never seen anything that has given me even a moment's pause. Stopping him from seeing them because of the vanishingly small possibility that they would both betray me in the most awful way seems a compete overreaction to the actual risk.
But if I did start to feel that something had change, that something was amiss, I would expect him to listen to me.
The reason I focus so much on the "new friend" thing is because it's such a cliche - you see it on here over and over again. And also because I think it is more risky - brand new exciting person, so much "getting to know you" intimacy to have. You don't really get that with someone you've already snogged years ago and who's seen you at your silliest.
My point in answer to the OP is that for me the correct answer to "do you allow your DH to go for dinner or travelling with female friends?" is "a qualified yes"
Yes, I am happy for him to do those things (well, the travelling isn't something that's ever come up) BUT I am not happy for him to do those things with any woman at all. "Friendship" can cover a multitude, including the early stages of a sexual relationship. And if that's what seems to be going on, then I get to call bullshit on "we're just friends"
The thing is, you only get 2 chances to call bullshit in an entire relationship - so you have to use them very wisely.
I still have both of mine fully intact. And hope to keep them that way for as long as possible. Maybe forever.