I'd have to agree. Everything with swinging comes with some risk, so as always it comes down to risk assessment and what your tolerance level of that is.
Nowhere near an expert an anything related to this, but I can't think of a much better way to transmit or catch a respiratory disease than sex. Take away the kissing (which would be a dealkiller for us right there), then think about the typical sexual encounter - enclosed space, man on top of woman, missionary, face to face, he's panting and breathing hard, she's panting and breathing hard, and this goes on for a while. Seems to me you almost couldn't come up with a better way of transmission if you tried. There's a huge IF there though, that only matters if one of them has it...which leads us right back to level of risk.
I guess you could dream up ways you hoped would reduce to some degree the risk, like no oral, doggie style or reverse cowgirl only, both be masked, etc. But for us it would be like the no kissing thing, when you start taking pieces away, pretty soon the whole thing falls apart and it's just not worth the trouble anymore.