In a way you have a relationship with a special relationship as opposed to a relationship with an individual person. That is you can have one wife and then another but both are called wife and are regarded in the same way and with the same level of affection.
I think that if you are imagining someone else.. While "making love"... Then you aren't making love, and you should leave that person. Waste of time to imagine someone else. Disrespectful. But that is just my opinion.
Well, it is disrespectful, and it makes me upset that people who are lucky enough to be able to find someone to be with, take that so much for granted.