Something about that, about feeling how much he's into it in the pressure of his fingers, makes Tony's breath catch.

Tony likes kissing. He likes physical contact in general and sexual contact in specific. He likes the simplicity of it, how it all makes sense the way few things ever do, how it puts him on a level where he can understand what's going through somebody's head the way he understands ballistics or subatomic particles or the melting point of iron. He likes it because it's fun. He likes it because it's fun you can share with somebody.

Why wouldn't he be grinning?