I've got lots of platonic friends, and I've never tongue-kissed a single one of them; not even to distract enemy agents chasing us down. (How does that "everybody hates a PDA" theory work, anyway? IRL, it's the total opposite: you see a couple making out in the middle of a crowd, and EVERYBODY looks; even if they're disgusted.)
Maybe you stare creepily at people making out in public, but I sure don't. And I'd wager a bet that most people wouldn't. Not because they find it disgusting, but because it's a very personal act. People avert their eyes because they don't want to be or feel involved in someone else's personal matters.
And that wasn't the only reason to kiss in that situation. Kissing was the only means of adequately hiding both their faces without looking suspicious.
The car ride immediately after the kiss sets up PLENTY of romantic and sexual tension between Cap and Natasha. That's the whole purpose of that dialogue. Especially the last lines of that scene:
Steve: You know, it's kinda hard to trust someone when you don't know who that someone really is.
Natasha: (eyes grow cold; then soften) Yeah...Who do you want me to be?
Steve (hesitates): How 'bout a friend?
Natasha: (smirks; then the coldness returns): There's a chance you might be in the wrong business, Rogers. (wistful smile goes away; they exchange a look)
That's the kind of sexual and emotional tension you can cut with a knife.
There was absolutely nothing sexual about that dialogue. Cap explicitly asks Widow to be his FRIEND and she completely rebuffs him.
If there was anything that brought sexual tension to that scene, it was the slightly knowing glances each actor gave to each other in that moment. But that struck me more as each of them trying to read each other than feeling attracted to one another.
I can also go on about how the majority of Black Widow's scenes in The Avengers features Cap as her personal knight in shining armor, and again, how she's paired up with Cap in Seoul in AOU while carrying his shield, but you'd probably go into denial about all that, too.
...what?
I can't think of a single scene in The Avengers where Cap saves Widow in any capacity, or where they even have any dialogue between one another that isn't purely business.
And are you serious about the Seoul thing? We have no idea about the context of that scene. And how does Widow holding his shield represent her emotional feelings for him in any way. Seriously. That's beyond reaching.
And just to put the Cap/Widow relationship thing to rest...she spent the entirety of Winter Soldier trying to get Cap a date with anyone OTHER than herself. And at the end, when she could have joined Cap and Falcon in their search for Bucky (spending much more time with Cap, who you believes she wants a relationship with), she leaves them to go do her own thing.
The Russos have said numerous times that Cap and Widow have a WORKING relationship and operate on the level of brother and sister.
Plus, you have the rumors of Widow being with Banner in AoS, and the potential for her to have a relationship with Bucky in Cap 3.
There is ZERO evidence to suggest that Captain America and Black Widow will ever be a couple. They just had an entire film that featured the two of them almost exclusively, with them being alone together in many scenes. Nothing romantic was explored.