I remember your thread too.
If your relationship is entirely platonic and you're both consenting and happy with this, and you treat each other with respect, does it matter if he's gay? Essentially you're just friends aren't you, married friends.
On the other hand, if the lack of physical relationship is an issue, that it itself is a reason to consider moving on, never mind him potentially being gay.
I know you're struggling to rationalise the message, but if someone has gone to the bother of sending you this note, I expect they have very good reason to believe he is gay. It's not the kind of prank that you'd take for a laugh, and given the other info you've shared it doesn't seem unlikely.
We have some friends where we, and many others, suspect he is gay. I've never mentioned it to her, and they seem happy but I do wonder if she's happy. They have separate bedrooms, at his instigation, for spurious reason.
Essentially; if you're happy with your relationship, ignore it, but if you're not you need to act to make yourself happier.