If it is just for momentary sexual satisfaction then it is unlikely to last long.
If it is to get a home and family then the tendency is for the relationship to last until children become more independent and then a sense of boredom sets in.
Earth (Practical), Water (Emotional), Air (Mental), Fire (Spirit. We come to this earth to learn spiritual lessons - self development. So I am more interested in studying the spiritual life lessons of INDIVIDUALS - which is based on our Sun Sign.
The more you relate your activities to your Sun Sign the easier life will become.
Also, relationships can be formed on different levels - such as for sex, common interests and hobbies, or business.