Generally on-screen “French” kisses, especially in TV or PG-rated movies, consist of the actors pressing their lips together while opening and closing their mouths. With their lips sealed together while doing this, what’s going on within their mouths is hidden from view. This presents the illusion that their tongues are meeting, but in fact they are usually not. It’s just an assumption the audience makes.
In more explicit films, the actors will actually entangle tongues. Even in those cases though, the tongue action is usually kept fairly tame. This is not necessarily because the actors would be uncomfortable — these people are generally professionals at doing things on-camera that would make most mere mortals uncomfortable. Rather, passionate French kisses between two stimulated people are actually rather messy-looking, and not the most romantic or picturesque things to see in detail on a big screen.
Here’s one example of an “explicit” Hollywood kiss (sorry, it’s an old favorite of mine):
As thorough as this kiss looks, seeing an actual up-close kiss between two lovers, with all the detail that a high-def camera would pick up, would generally look nothing like this. For the sake of cinematography, Hollywood gives us a much friendlier fantasy take on what kissing is.
In their defense, I think directors and actors seek to portray how nice kissing feels for the people engaged in it, rather than presenting a realistic version of what it might look like to an onlooker, which I think is the more important priority in most stories.