A long time ago, I remember being out with a female friend who told me she occasionally fantasized about other men when having sex with her boyfriend. At the time, I wondered how common this is among couples. I also wondered how many people do this but don't admit it to others, as my friend had.
There are two issues here. First, is it okay to sexually fantasize about others? Second, is it okay to sexually fantasize about others while you are having sex with your partner?
Let's take the first issue on its own. When I see couples in couples therapy, the couple often deals with jealousy issues. I explain to each member of the couple that no one person is going to meet every need the other has, sexual or otherwise. I encourage the couples to learn to allow each to remain an individual inside the relationship, and accepting that your partner may fantasize about others can be very healthy. You give your partner a certain freedom in doing so. You don't put pressure on your partner to only be attracted to you.
The second issue, I think, is more complicated. Certainly no man or woman wants to be in the middle of an intimate moment and wonder whether their partner is actually picturing someone else in the moment. After a couple has been together many years, it's natural for each member to have a fantasy about being with someone else. However, it's not natural to have this fantasy while you are in the act with your partner on a regular basis.