Technically this is a form of exhibitionism rather than "group sex" per se.
A rainbow party is a baseless urban legend spread from the early 2000s.
When a person is entered or penetrated in the vagina and/or anus by two people at the same time.
This usually is when one enters the anus while another enters the vagina, although it does also refer to two simultaneous penetrations in the same orifice.
In principle, any sexual behavior performed by more than two people can be referred to as group sex, but various terms are used to describe particular acts or combinations of people.
Many swingers argue that non-swingers have conflated the terms because of lack of understanding and that there are distinct differences among the terms with specific meanings as to number, intent, sexual orientation, and familiarity of the persons involved.
Group sex most commonly takes place in a private sex party or semi-public swinger gathering, but may also take place at massage parlors or brothels or, in some jurisdictions, at purpose-built locations such as sex clubs.A sex party is a gathering at which sexual activity takes place.Sex parties may be organized to enable people to engage in casual sexual activity or for swinging couples or people interested in group sex to meet, but any gathering where sexual activity is anticipated can be called a sex party.Since the directors could not show penetration they had to figure out new, visually appealing ways to approach sex acts that would satisfy the audience without violating Japanese law.
As with all sexual activity, the relative risks of group sex depend on the specific activities engaged in, although having a large number of sexual partners increases one's risk of exposure to sexually transmitted infections (STIs).
Sex parties, under various names, have been a common focus of moral panics fed by media reports claiming that such parties are prevalent, or growing in prevalence, especially among teenagers.