Too early "boyfriend"/"girlfriend" talk

(Mini-post below this one.)

You all know my older son is in Kindergarten (he's turning 6 in less than a month).

From the beginning of the year, he and the other boys were talking about which girls they were going to marry, and stuff like that. I didn't think much of it, because they're surrounded by married people, so it all seemed like a game of pretending to be grown-up. (I also enjoyed the way their wants outpaced both legal reality in the US and logistical comfort everywhere, like when they decided that two boys and two girls would all marry each other and live together. It makes the argument about which way the toilet paper should roll look like fun.)

But a month or so ago, he started calling one of the girls in his class (we'll call her M, because that's not her real initial) his "girlfriend." M told my babysitter that they kissed on the playground(!) and my son has been concocting elaborate plans for their future life together.

I have no idea what to make of all of this. I appreciate the playing at being grown-up aspect of it. But it just seems waaaay too early for them to be talking about being boyfriend and girlfriend. I'm really uncomfortable with early sexualization of children (remember this post about toddlers flirting?). But it's clear that my son doesn't really understand what being boyfriend/girlfriend encompasses for adults (or older teens), and just means that he likes M a lot and likes to play with her.

So, how do I talk to him without freaking out and telling him he can't have M as a girlfriend when M is really just his "girlfriend"? It seems dumb to forbid him from doing something he's not actually even aware of yet. This is all just confusing me.