I agree. I don't think I could deal with a don't ask don't tell arrangement with a lover or a metamour.
I had a very brief exposure to a guy from OkCupid in an open relationship a bit over a year ago. His wife seemed reluctant to talk to me. My newbie mistake was an email where my intent was to make sure the relationship was actually open....but I didn't do it well, she didn't take it well, and the relationship with the guy never progressed past IMs.
She didn't want to be forced to acknowledge me. She didn't want her husband to actually have a relationship with me. If I had been just an anonymous fuck she'd have been happier, but, I can't do that sort of thing.
So, better to know right off the bat that a DADT situation is a bad fit for me.