Mostly depends on who you kiss, I guess.

The New Yorker: The Talk of the Town:

Tim Wagner, a spokesman for American, said that the stewardess’s injunction to the men was reasonable, and would have been made whether the couple was gay or straight. “Our passengers need to recognize that they are in an environment with all ages, backgrounds, creeds, and races. We have an obligation to make as many of them feel as comfortable as possible,” he said. (He added, “Our understanding is that the level of affection was more than a quick peck on the cheek.”) But a customer-service representative named Terri, reached last week on the telephone, offered the opinion that kissing on airplanes is indeed permissible. “Oh, yeah! Sure. I’ve seen couples who are on honeymoons,” she said. “They just don’t want you to go into the bathroom together.”