antimarital sex

Sex that happens between people that don't ever get married (to each other or otherwise).

Premarital sex is to sex that happens before marriage. Antimarital sex is to sex that happens in lieu of marriage.

Sex that is done by a person that never gets married.

*At Betsy's funeral*

Jim: Betsy should be in Hell because of all the premarital sex she had.

Dave: Betsy should be in Heaven because she only ever had antimarital sex. You see, she never did get married.