An agreement reached between at least 2 people meaning that in the case one of them fails to get lucky, the other one will provide them with the desired service (usually intercourse).
Having someone ready to fall back on should one not be able to get laid. This can be a friend, a f*** buddy, a partner in an open relationship, a sex acquaintance, etc. Problems often arise when 2 people both have sex insurance with each other, and only one gets laid, or one takes out too many sex insurance policies ie. sex insurance with too many people.
1 - Amanda had a hot date last night, but the loser said that he "had to be up early" the next morning and left her without any action. Luckily Amanda had agreed with her friend Steve that they would provide sex insurance to each other, and since Amanda was in the mood, she called Steve up when her date left and they slept together.
2 - Mike and Ted happened to not pick up any chicks at the bar last Saturday. Luckily for Mike his sex insurance partner also had no luck that night and he called her on the way home to hook up with her.