Very often people do misunderstand the term "full body massage". It doesn't mean to massage every single square centimeter of skin. It refers to a full massage, front side and back side, including head and feet. Due to professional ethics, the sexual zones, are never touched. A professional full body massage will never lead to sex. The only massages that lead to sex are the sensual massages that are received between partners for the sole purpose of sex. Those massages shouldn't be referred to as "full body massages," as the professional Massage Therapist uses this term for a typical massage.
A full body massage either starts on the back of the body or on the front. To guarantee an exceptional body treatment, a therapist will either start at the feet and work herself up to the head before turning the client, or she will start at the head and work herself down to the feet. The best starting point is always the head by having the client lay on his or her back. Only the massaged body part is uncovered; the rest is covered up to provide the person security. When starting with the head, the person will be able to see you and build trust, some important aspects to find relaxation and to start the process of rejuvenation.
A full body massage will only lead to sex if the person giving the massage will touch sensitive areas such as the breast, the vaginal areas or the area immediate to the sexual organs. During a professional massage, those areas are never to be touched, as they are not part of a therapeutic treatment. If the person giving the massage is offering the massage in the hopes of turning it into sex, he or she will know how to go about. The person receiving the massage will also feel his or her energy and will either respond by relaxing and enjoying it or by letting the person know that she or he just really wanted a therapeutic massage.
But to explain why massaging could lead to sex is very simple. If our goal is to have sex, then we most often will engage in foreplay. Sensual massages are an extended version of foreplay. Touching the skin very softly will stimulate the central nervous system. This will give the person a feeling of wellness and send them into a deeper stage of relaxation. Being massaged gently doesn't necessarily create a desire for sex. The sexual desire is awaken when sensitive areas around the sexual organs and chest are touched and massaged. It is also the way somebody gives the massage, maybe running the fingers through the hair, down the side of the neck, further down the side of the body. This would stimulate the sexual hormones, leaving them wanting more.
So why do full-body massages always lead to sex? The giver won't be able to distinguish between a professional massage or a sensual massage. To him, massage means foreplay. As he touches the soft skin, he will get aroused. The person receiving the massage is looking forward to being massaged, as it will release the good-feel neurotransmitters (endorphins.) Unless she is only looking for a therapeutic massage, she will enjoy the gradual increase in stimulation which will then lead to sex.