have relations with

have (sexual) relations (with someone)

To engage in sexual activity (with someone); to have or be in a sexual relationship (with someone). As there was no hard evidence that Mr. Smith's ex-wife had sexual relations outside of marriage, the judge was obliged to uphold the prenuptial agreement during the divorce proceedings. I hear Jim got fired for having relations with his secretary.
See also: have, relation
Farlex Dictionary of Idioms.

have relations with someone

Euph. to have sexual relations with someone. While engaged to Mary, he was having relations with at least two other women. She was having relations with one of her employees, which was strictly against policy.
See also: have, relation
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • have (sexual) relations (with someone)
  • relation
  • sexual
  • sexual relations
  • piece of snatch
  • (but) not in the biblical sense
  • biblical
  • get (one's) ashes hauled
  • piece of tail
References in periodicals archive
Meanwhile, a senior Kuwaiti official said that his country would be the last Arab country to have relations with Israel.
The Kuwaiti position has been clear since the late Emir Shaikh Jaber Al Ahmad who ruled from December 1977 until January 2006 said that Kuwait would be the last state to have relations with Israel, "with respect for all statements that referred to the possibility of taking steps to normalise relations with Israel," Al Jarallah added.
Netanyahu lands in Chad to restore relations
In spite of the negative connotation, many men say that it is not necessary to use a condom with their partners, since "the women are faithful and only have relations with them"; thus, there is a double moral standard in sexual practices of some men.
A gendered perspective on men's reproductive health