fraternize

verb

1. (intransitive)To associate with others in a brotherly or friendly manner.
2. (intransitive)To associate as friends with an enemy, in violation of duty.

   “Now in this world it is never wise to meddle with such things. It's better to leave them as they are. No one ever had any good come to him from fraternizing with too great familiarity with the unknown; and it's a very good thing that the children from The Other Side are as shy as they are, as Alice found out through Evelyn Dearie.”

3. (intransitive)To socialize in confidentiality.
4. (intransitive)To have an intimate or sexual relationship with a forbidden party.