Thank you for coming to Alterheros with your question.
In your question, you explain that you are happily in a relationship with your boyfriend and are thinking about engagement and marriage. You also bring up that you sometimes fantasize about other women during foreplay but not during sex* with your boyfriend. This has caused you to feel some guilt, being as you desire to stay with your boyfriend and eventually get married to him. You ask the question why you have to fantasize about other women.
Human sexual feelings can be very complex. It is understandable that your fantasies about women can be new and perhaps frightening territory that you haven't experienced before. It can be especially troublesome as you're in a committed relationship. Rest assured that having fantasies about being with someone else doesn't mean that you are no longer attracted to your boyfriend or that you should have a sexual experience with someone you fantasize about. Fantasies are sometimes no more than that – fantasy. Sometimes we fantasize about things we would never actually want to do if given a chance in ‘real life’ situations. But these CAN favor sexual stimulation.
Also keep in mind that it is normal to not experience an orgasm during sex*. Many people may not reach orgasm during each sexual encounter, while some have many orgasms. Do not feel the need or the pressure to have one during sex* with your boyfriend.
*I would like to note that many people have many different interpretations as to what the word sex means. For example sex' can mean: vaginal intercourse, fellatio, cunnilingus, anal penetration or other acts to different people. In my answer, I try not to assume one act over the other in the use of the word sex'.
Thanks again and do come back with any further questions you may have.
All the best,
Kelley for Alterheros