OK, so you're a lot younger than me! That episode premiered in the US in February 1993 and didn't arrive here in the UK until June of 1998, by which time I would have been forty, my daughter twelve and my son eight.
Jen was in senior school by then, and remarked that a lot of the older kids had 'done' Valentines but not many of her classmates. Rik was mystified by the whole thing! Not that either of them was an innocent, Jenn was an avid fan of Kevin Sorbo as Hercules and Rik would sit glued to Xena: Warrior Princess.
It's just that Valentines' here is more restricted - basically, unless you are in a romantic relationship with someone, you don't send or get a Valentine, and to get more than one in any given year is to risk your reputation, big time! Lisa giving Ralph a Christmas card because nobody else did would make more sense over here, but that would've been a different story!
So that's probably why the idea of Primary School kids exchanging Valentines creeps me out! But it does worry me that kids that age are shown as even thinking about the whole boyfriend/girlfriend thing in any other context than a reason for the ridiculous behaviour of older siblings!