One of the husband's Torah level obligations to his wife is to have relations with her at regular intervals (though she can waive that right on any given night). The Talmud discusses different intervals, depending on the husband's profession. The minimum interval for a talmid chacham (Torah scholar) is once a week and the Talmud recommends that that once a week be on Friday night. This is codified in Shulchan Aruch Even HaEzer 76:2.
So, while it is not an absolute halachic obligation for couples to have relations on Friday night, it is very strongly encouraged.