Amito Fo,
In Buddhism, ultimately the goal is to become a Buddha. Buddha's do not have sex. So I will explain a bit about this Precept (rule) so you can more quickly bcome a Buddha.
If you are a Buddhist Monk the rule is simple, No Sex of Any Kind is allowed. This rule is the #1 rule in the Vinaya. The Vinaya are the rules which the Buddhist Monks must follow. They tell us who is a monk and who is not. If a Buddhist Monk breaks this rule they lose their Monkhood and are no longer considered a monk.
If you are a Layperson only sex between 1 Man and 1 Women in Marriage is allowed. All other kinds are considered Inappropriate Sex and are a violation of the Precept. This rule is 1 of a group of 5 common Precepts that lay people may take.
Some of the kinds of Inappropriate Sex not supported by Buddhism are
Man + Man - Gayism
Women + Women - Lesbianism
Man + Women (Outside of Marriage)
Bisexualism
Multiple Partners at the same time
Sex with Animals (Beastiality)
Self sex
Amito Fo,
Enjitsu

