"Sexuality is a dynamic expression of your psyche more a state of consciousness than a biological drive. The quality of attention and togetherness achieved during sex allows you to engage with your partner at a deep level of intimacy. Focus on opening your heart so that you can feel the love within your heart pouring out, and enjoy a deep sense of connection both with your lover, and through them with the rest of the world." (Cassandra Lorius)
Naturally, the answer to this question is dependant upon whom you ask. Of course, sex can be a pure physical thing that is driven by our primitive instinct to reproduce, but that is not the good' sex that we all need, want and seek. That is mental.
Studies have shown that sex for men is more of a physical and visual experience. Sex for women is more of a mental or cerebral experience. It has been argued that we are hardwired this way, however, our attitudes and behaviors associated with sex is strongly influenced by our upbringing, early sexual experiences, and our ability to feel. Physical sex does not require the emotions, feelings, and vulnerability that mental sex does. For that reason, many people never move beyond the physical act of sex.
Ultimately, all sexual behavior can be reduced to being a "mental thing" since arousal must first begin in the mind. A man may get an erection from looking at something that turns him on or from being touched in a sexual manner and these are natural biological reactions to stimuli (physical). But, it is the thought (mental) that he has first in order for his body (physical) to respond. Whether conscious of it or not, sex is happening in the mind and brain, not just the body.
That is not to say that after the mind has been aroused that sex cannot be a total raw lustful physical experience. I believe these interactions are necessary for people who are afraid of the intimacy involved with good' sex. Intimacy requires feeling, trust, letting go, and being vulnerable. It can be scary trust, be vulnerable and release yourself fully with another human being, especially sexually. This fear, conscious or unconscious, of intimacy prevents many people from ever experiencing a deep connection with another human being and from exploring their full sexual potential. Because of the intense feelings and emotions (mental) associated with intimate sex many people have one physical experience after another believing that it will give them the blissful sexual connection that only intimate mental' sex can provide.
In the end, sex is a physical act, but it is experienced mentally. Anyone can perform sex, but not everyone can feel' sex (mentally) since this requires a level of intimacy that few people explore, let alone experience. It can be terrifying to open your heart and mind and allow yourself to pour out to another human being, however, it is necessary if you wish to experience the bliss, connection, and intimacy with good' sex.