Very true, but I don't think it has the negative connotations or the included subjection/humiliation that cuckolding has. Granted, voyeurism only used to be used to describe "Peeping Toms" or people who spied on others, not people who were willingly included in the room during sexual acts. Now it is used for more inclusive things, though, and it's even used for nonsexual experiences like reality tv and such.
I don't know that there is a perfect word for it, but I would think voyeurism would be the closest thing since it is getting pleasure from watching others. Cuckolding has the humiliation aspect that I don't think is included in most people's enjoyment of watching people (significant other or otherwise) have sex.