Do you believe a woman when she tells you she truly is only a friend with an ex?

Nope

It's not you they are worried about, it's the ex.

It´s possible but not easy.
I would trust my wife (if I had one), of course.
But the ultimate truth would not come from the labels: "best friend", "Ex", others.
Would be, no interpretations needed nor accepted, on how he behaves.
Never a matter of feelings nor intentions: no grey facts.

It may help, and a lot, that your Ex would become a friend of, not only or mainly your SO, but a loyal friend of the couple.

I'm ok with girls maintaining relationships with or "seeing" their exes as long as they aren't abusive.. Especially if they were good sexually

if the relationship is in a rocky place I'd advice against them seeing an ex. But if the relationship is good why worry about it?

Yes because you can see the anger when it’s the truth. You know there is nothing more.

They’re just a wind Change of direction away form doing it...

Does this include sex with the ex? Or you meet in a public place?

Sure, and I'm best friends with my ex too.

I’ve been burned before with the “ex”.

If you dont have trust then you dont have anything

Yes. I have one myself.

I don't believe so.