Is it? Legality aside, if you define the limits of unacceptable behaviour then you're also saying that anything outside that is acceptable. If you hire decent human beings, hopefully your team can use their judgement and empathy to tell whether something's OK or not, rather than "S/he shouldn't have been offended, I wasn't breaching the rules laid down in the HR handbook! Grow a pair!"

(On the flipside, if you hire decent human beings then nobody's going to be looking in the HR handbook to determine how far they can push harassment without being fired anyway.)

Writing rules to dictate the personal relationships of your employees is most certainly worse than not. It's draconian and insulting.