I always watched. It was part of our agreement from the start that if we were to indulge in the lifestyle, then I would be participating (if only by being present in the room).
While it was weird the first few times, it was not necessarily so for us. Her Bull was a college kid, much younger than either of us (an added turn-on for me - watching my wife getting her brains fucked-out by some hung punk jock), and while at first he was slightly put-off by my watching, in virtually no time at all he got over it, and began to actually enjoy plowing her in front of me.
It was only after a few months of them being together and my watching that she decided she was "uncomfortable" with my being present, and I was relegated to having to sit outside our bedroom door while they got it on in our bed. An AMAZING turn-on, though.