Some people are willing to fantasize about things that others aren't. They may find it pleasurable to fantasize about sex with their partners, other people's partners, animals, all sorts of combinations. Other people think it's wrong to fantasize about sex at all, about certain kinds of sex, or about sex with certain people. Men are much more likely to fantasize, or to admit to fantasizing than women, which is what one would expect in a society that encourages male sexuality, but only recently has begun to accept female sexuality. There is enormous variation in the amount of fantasizing that individual men do.
Some men have sexual fantasies many times each day, while others can go for weeks without one.
Often men who prioritize sex in their lives have more sexual thoughts and fantasies. Remember in thinking about your boyfriend, that if he does fantasize sexually, it in no way means that he is following through on his fantasies or that he loves you any less. It means that he is using his imagination, combined with real life sexual stimulations, to turn himself on. Whether or not you discuss sexual fantasy with your boyfriend is your decision. Potential deterrents include the risk of upsetting your partner, jealousy, rejection, and hurt feelings.