The picture showed a happy man and woman sitting on a couch and playing Scrabble, their wine glasses nearby. "We bought an apartment together. Maybe we'll get married. Maybe we won't. Anyway, it's a good investment," read the advertisement for the cooperative apartments at River Place in Rosslyn.

What the full-page ad in Saturday's Washington Post failed to point out, Arlington law enforcement officials contacted yesterday said, is that Virginia law makes it a crime for an unmarried couple to "lewd and lasciviously cohabitate."

"I see nothing in the law that would prohibit the ad," said Arlington Commonwealth's Attorney Henry E. Hudson. "But if people follow the living arrangement hinted at in the ad, they may very well be susceptible to prosecution.

"This is not exactly a high investigative priority . . . ," he said. "We would prosecute only if someone brought charges."

The River Place ad brought a handful of complaints to the Post advertising department. Robert M. McCormack, Post vice president for advertising, said the newspaper would not knowingly carry an ad advocating illegal behavior.

River Place developer Gary Nordheimer said he was surprised that anyone complained. "We don't condone or preach lewd and lascivious behavior," he said. "But we have recognized, by today's standards, that lots of single people do live together. We're not preaching it, but recognizing it. We're taking a very realistic approach to the market."

Because of negative reaction to the ad, however, Nordheimer said it will not run again. "I think in the coming weeks you're going to see this couple get married [in the ads]."