You never know. It might not be true. Also, the world was very different in the late 70's / early 80's.
I don't really care much about Christopher Reeves's sexuality in particular. Whether he had something to hide or not, I don't know. The article just makes me feel sad that so many people have felt pressured to keep parts of their lives secretive.
Actually I think most actor probably are bi I don't know about them being straight out gay but if you have a tone of money and have you pick of the most attractive people then I think most people would experiment at least a little bit. He probably was just curious.
It's entirely possible. Not long before his accident he came to a farm where I worked to look at a horse that was for sale. A large percentage of men in the equestrian community are either gay or bi, and nobody really gives it any thought anymore.