Well, I top other lovers at will and there's a certain amount of bottoming I can do, depending, but I am owned by him alone. The power dynamic in our relationship extends past our sex life, which is not the case in other relationships -- even if I let another lover spank me or tie me up during a scene, that doesn't make me their property, and I don't get involved with people who can't respect the fact that my husband/owner has final say in whatever I do, with him or with other people.
Some close friends have a similar situation; they are in a relationship where the sub may sexually dominate other lovers if he wishes, but is not to submit (sexually or otherwise) to anyone else but his dom, who retains the right to determine what his sub can or can't do in the bedroom.
Thanks for the welcome, everybody!