You could be right with it being a premonition. If not your future than perhaps someone elses experiences either in the present or if the mattress is previously used then an impression left behind from anothers experience.
It is natural to have these types of dreams, just because there is no conscious desire for intimacy doesn't mean that there isn't a curiousity about it at some level. Possibly a need for something (not necessarily sex, but maybe challenge or excitement), or it might be representing your comfort level with where things are in your life.
I've added the blurb offered by Dream Moods on the topic. I was considering editing it for your needs, but then there might be others who are a little embarased about posting info about their dreams. So I'm leaving it as is. Hope it proves to be useful.
http://www.dreammoods.com/dreamdictionary/s.htmSex
To dream about sex, refers to the integration and merging of contrasting aspects of yourself. It represents psychological completion. You need to be more receptive and incorporate aspects of your dream sex partner into your own character. Consider the nature of the love-making. Was it passionate? Was it slow? Was it wild? The sex act parallels aspect of yourself that you wish to express. A more direct interpretation of the dream, may be your libido's way of telling you that it has been too long since you have had sex. It may indicate repressed sexual desires and your needs for physical and emotional love. If you are looking for a place to have sex, then the dream may be analogous to your search for intimacy and closeness. You want to rekindle some relationship. If you dream of having sex in a public place, then the dream implies that others are talking about your private relationship.
To dream about sex with someone other than your spouse or significant other, suggests dissatisfaction with the physical side of your relationship. On the other hand, it may be harmless fantasy. In such situations, you may find that you are less inhibited sexually. Perhaps you need to bring the same sense of adventure into your existing relationship.
To dream that you are having sex with an ex or someone who is not your current mate, denotes your reservations about embarking in a new relationship or situation. You may feel nervous about exposing yourself and are feeling a resurgence of those old emotions and feelings that you felt back when you and your ex were together. If you are approaching your wedding date, then is not uncommon to experience especially erotic adventures with partners other than your intended spouse. This may be due to the intensity of the sexual passion with your fianc�. It also relates to the new roles that you will be taking on and the uncertainty that that may bring.
To dream that you are having sex with a stranger, represents uncertainty about what is ahead. Alternatively, the dream allows you to experiment freely without having any hang ups, emotional baggage or preconceived notions associated with a person you would know. In such a scenario, you are able to let loose and express your desires, passions and emotions. In particular, if you dream of having sex with multiple partners at the same time, then it indicates that you are feeling detached in your personal relationship. You feel that sex is just an act devoid of any emotion or passion. Perhaps your sex life has become too automated.
If you are heterosexual and you dream that you are having sex with someone of the same sex, then it represents an expression of greater self love and acceptance. You need to be in better touch of your feminine or masculine side. The dream does not necessarily imply homosexual desire.
To dream that you are having sex with a celebrity, indicates your drive to be successful. You are striving for recognition. Consider what movies your associate this celebrity with for clues as to where and what you want to achieve success in.
To see your parents having sex in your dream, indicate that you are seeing similar aspects between their relationship and your current relationship. Most of us cringe at the thought of our parents having sex, so this dream imagery is really trying to get your attention. Consider your parents' real life relationship together and what you can learn from it.
To dream that you are the opposite sex, suggests that you need to incorporate certain qualities of the opposite sex. Ask yourself, how do you feel being a man or a woman? In what ways can you incorporate those feelings into your waking life.
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