It's not a first for the series, the left behind DLC had riley and Ellie kiss. Also, you do know human beings kiss, right? it's not some sort of egregious gesture of love, they aren't fingering each other on stage. I guess if you see it as a form of Public display of affection and you're not comfortable with it, then I see your point.
I thought the Riley Ellie kiss was fine. Melded well with the story and was sweet. I thought the trailer was shot to maximize social agenda impact.
Edit: and for that matter - it worked, as I saw multiple sites reporting on "That" kiss.
Owner of PS4 Pro, Xbox One, Switch, PS Vita, and 3DS