Buddhists try to conduct themselves in a way that will not cause suffering to others and try to always be caring and loving.
The Five Precepts are important for Buddhists when considering their attitude to sex. The third Precept states that Buddhists should not engage in sexual misconduct. This might include adultery, as being unfaithful to a partner can cause suffering, and promiscuity, which can be seen as a negative expression of craving after sexual stimulation. Sex should form part of a loving relationship (eg marriage).
Buddhists can use contraception as long as they have the Right Intention. Good motives such as responsible family planning and disease control may be accepted. Indeed, if sexual intercourse may result in an unwanted pregnancy, such suffering needs to be avoided and contraception should therefore be used.
The Buddha taught that in many areas of life, depriving yourself of something or over indulging in something leads to suffering and dissatisfaction. He taught that The Middle Path should be followed. This can be applied to sex.
Buddhists are encouraged to enjoy sex responsibly, as a result, most Buddhists avoid being promiscuous. Chastity is not a requirement of leading a Buddhist life.
Buddhists do not see marriage as a duty and cohabitation is perfectly acceptable. As long as neither partner suffers, a Buddhist can enjoy a healthy sexual relationship.