From Longman Dictionary of Contemporary Englishsleep together phrasal verbSEX/HAVE SEX WITHif people sleep together, they have sex with each other → sleep→ See Verb table
Examples from the Corpus
sleep together• If it is possible, we will sleep together.• Leith's mouth fell open in surprise as she realised that this awful man thought she and Travis were sleeping together.• For ten days we ate together, slept together, never spent two minutes apart.• She had obviously enticed Patrick to run away with her - were they sleeping together, she wondered briefly? 