Is it permissible to bury a male and female side by side in separate graves if the 2 are unrelated? Yoreh Deah speaks of this but I do not know if this question is covered.
There is no halachic problem in burying a man next to a woman.
Some try to be particular for this, but as noted there is no halachic problem here.
See: Kol Bo on Aveilus (p. 179) who writes that there is no problem in this, and adds that this is the common custom in many communities; Shut Imrei Eish (Yoreh De’ah 117) writes that there is no “disgrace” in the matter, even for a great Torah scholar to be buried between two women (one was his wife and the other a stranger); Shut Peri HaSadeh (3:121) likewise writes that there is no problem in the matter.
See also Shut Tzitz Eliezer (Even Yaakov no. 28) who writes that this is fine, and this was corroborated by Rav Moshe Shaul Klein in the name of Rav Shmuel Wosner, though he add that one can designate a special area for those who wish to be particular in the matter.