This is true on a number of levels.  First, there has to be the original turn-on, which can be a thought, a sight, a person, a picture, almost anything.  The physical reaction comes after the mind engages into gear, just as we act in reflex if we touch something hot by pulling the hand away in a split second.

In the same way, a man may have a physical reaction, that is, an erection, to seeing a beautiful woman — but this happens a split second after his mind registers that this is 'exciting'.

Conversely, if sex is not in the mind, and if there is no mental stimulation, either sex is very mechanical or the body may not perform at all, as in the case of 'distraction' causing sexual dysfunction.  Even during the sex act, mental imagery is important to the continuation of stimulation.  Most people use fantasy as a common method of arousal and for maintaining interest during sex play and intercourse.  I'm not suggesting that being with a loving partner is insufficient, only that the mind ultimately controls sexual arousal, behavior and satisfaction.

This becomes clearer when we look at the role of emotion in sexual exchange.  Many men, and some women, maintain that emotional involvement is unnecessary for sexual fulfillment.  In other words, you don't have to be in love to enjoy sex fully with someone.  If we look at what happens when there is emotion present, the difference is immediately apparent.  Sex then becomes multi-dimensional.  It engages the mind, the body and the heart, thus transforming into a more complex, and ultimately more fulfilling experience.

However, it can work against some lovers who find that the presence of emotion creates anxieties that are absent when it's just sex alone.  For example, some men who suffer from impotence tell me that when they're with a partner they are not in love with they have no problems.  Some men and women suffer from a variety of behavioral disorders when they feel emotionally vulnerable during or after sex.  This can manifest as coldness and distance straight after sex or, in more severe cases, quarrelling and violence the next day.

Emotion, of course, is not only love.  There can be various forms of anxiety associated with sexual feelings, such as emotional vulnerability, fear of failure, performance anxiety, psychological blockages, past hurts, lack of trust, relationship issues.  Because we are human beings and not machines, we cannot discard the armoury of all our feelings and frailties when we enter the sexual arena, no matter how turned-on we are.