I’m a single, virgin, 26 year old male, but I can honestly admit that if I was having intercourse with my wife, and that particular night of love making isn’t turning out to be as pleasurable or as unitive as we would both like it to be (ie- something’s just off, not enough that we don’t want to make love, but enough that the sync isn’t there or something has confused our senses that night, whatever), then I think that in order to get the juices flowing and the lukewarm passion of the evening heated again, that it is ok to fantasize about your husband or wife, provided that the fantasy is in keeping with their actual personality and doesn’t have them doing something awful or against their nature.
If a wife would get more into the sex by imaging her husband kissing her body in ways he would normally do on a great night of love making, then that seems fine to have it. Likewise, if the wife does this thing for her husband that really turns him on, but it’s not happening this night, he might fantasize of her doing what she’d normally do. I don’t personally see a problem with imagining your spouse doing something they already would do because it’s not physically happening yet. If after the fantasies you both get more into it and things start happening like they normally do, then common sense would say you don’t need the fantasy anymore, since you’re getting the real thing.
Of course, others here have given different circumstances (like your spouse being a burn victim or extremely overweight). In cases like this, if it’s so bad that you NEVER want to have intercourse with your spouse, then I don’t think you’re obligated to even if fantasizing would be considered ok no matter what in order to make the act happen.
There’s another danger here in having sex when you don’t want to- you rob your spouse of their dignity. They know that you know you’re only having sex with them with the aid of fantasies and imagination. To have their body used in this way is just as damaging than simply not having the sex to begin with would be. You’re darned if you do and you’re darned if you don’t. If you don’t have sex, he or she feels unattractive and unworthy. If you do have sex and you’re repulsed at having to do it, then they feel the same- my husband/wife is only doing this out of pity and to try and keep the marriage bed together, but it’s just a show…they don’t want to do this and they’re forcing themselves.
It’s sad either way, but I don’t think imagination solves or answers the question, it just creates more questions. I’m not sure how to answer this any further.