If you are of the opinion that one excuse to tell dirty jokes is as good as any other, and the ABC Television Network apparently is, then "The Ropers," which premieres a limited six-week test run tonight at 9:30 on Channel 7, will probably not seem much more offensive than "Three's Company," the show off which it is a spin.

But at least "Three's Company" has something that might generously be termed a premise, albeit one stolen from a satire on TV pilots that Albert Brooks did a few years ago for "Saturday Night Live." The premise is that two gals and a guy live together in a platonic bubble that one libido or another is continually trying to burst. The program is television's first exercise in triphonic double entendre.

"The Ropers" are the married couple who live downstairs on "Three's Company." In the premiere of their own insanely unnecessary new series, they sell the appartment building to buy a townhouse. The preoccupation of the show's writers, however, is with sexual frustration, from which the characters are perpetually announcing they suffer.

So when Mr. Roper says to his wife, "Why can't you take 'no' for an answer?" Mrs. Roper replies, "I have, three times already this week -- last night, and the night before that," and so on. Other than the fact that they are both repulsive and hideous, and played by Norman Fell and Audra Lindley with such an abundance of mugging that you expect them to start chewing on the camera lens, these two characters have no identity whatsoever.

They exist purely to make jokes about one another's sexual appetites. She seems to have a ravenous one and he seems to have none. And in a burst of Bel Air inventiveness, the writers team them with another couple with precisely the same problem.

"Did you come home for lunch, or five minutes of fun?" the realtor's wife asks him expectantly. "Ann, don't arouse the beast in me during business hours," the blad stuffed shirt replies.

It is one thing to imagine you are reversing sexual stercotypes and another to dwell on this cheap trick with the obsessiveness of Heathcliffe prowling the moors. When the writers try to out-stoop themselves and toss in a few joking references to the sexual molestation of children, "The Ropers" sets that new low that ABC always seems to be after.

One thing the program does make clear is that there still is a difference among the networks -- only ABC would run a program like this.