I think it's a shame we have to spin such extreme stories about what, to children, is just a bit of fun. Put up and shut up, indeed!
My 5 year old got a card from a girl in his class. So did the other 23 boys and girls in the class, and it seems she was the only little girl who really had an interest in it this year! He gave me a heart made of clay, because he loves me more than Lego Ninjago. There's a lot of imposition of adult views of romantic love on kids going on in this thread. I'm particularly intrigued by the notion that a little boy giving a little girl a Valentine isn't play in the same way pretend shopping or baby care or construction isn't.
Why not? To a child, all play is "real", children don't make that distinction between acting with a doll and what they do in "real life" in quite the same way as we would, even if they might push boundaries more with a baby doll than a baby sibling, say. When we were kids, playing at getting married was a great theme, and we would sneak into the church with posies of daisies and put tissue paper veils on our heads and pretend to be grown ups... getting married. Girls got married to girls and boys to boys sometimes, girls to boys too... we had teddy babies that we shoved up our sweaters. It was absolutely innocent and beautiful, there was nothing to worry about or feel should be discouraged.
I only ever got Valentine's paper cards, usually a sheet from an exercise folded in two with a heart cut out, at school, and then a few in the first years of my relationship with dh (hadn't ever been with someone around Valentine's before that relationship). We knocked it on the head after a few years as we just don't go in for that kind of thing.