A charming widower (Robert Mandan of Soap') and his son (Randolph Mantooth) fall in love with the same girl (Cathy Lee Crosby); an easygoing bachelor (Roddy McDovali) becomes allergic to the idea of marrying his aggressive girlfriend (Tammy Grimes); and Isaac and an influential ex-shoplifter (Debbi Morgan of "Roots: The Next Generations") on the ABC Television Network's "The Love Boat," SATURDAY, JAN. 27 (8:30-9:30 p.m., EST). From the moment Woody Sillingsley (Mandan) is introduced to his son's new girlfriend, the two hit it off too well to suit young Billingsley, in "Like Father, Like Son," written by Jerry Winnick. In "Don't Push Me," written by Barbara Evans, Christine (nis. Grimes) surprises the bachelor by joining him on the cruise, then mistakenly believes he has proposed marriage. In "Second Chance,'' written by Lee Aronsohn, after Capt. Stubing has given the teenager a job in the ship's gift shop through a juvenile rehabilitation program, he sees her alone with earrings that later disappear.