So, all the real couples have kissed: Penny & Leonard, Sheldon & Amy, Bernie & Howard.
But who else, either in reality of fantasy?
Amy kissed Penny once in a bar.
Raj kissed Howard once, when he was aiming for Bernie, and Howard ran interference. (There was also that thing with the virtual device.)
Sheldon kissed Penny in Leonard’s dream.
Raj kissed Bernie several times in his own (on-screen) fantasies.
Am I missing anyone?
Leonard kissed Priya, Stephanie, and the artist babe.
Raj & Penny ended up in bed at one point. They probably kissed.
Beverly kissed Sheldon. Mary presumably kissed Leonard’s father, and the guy she was with when Sheldon saw her going bouncy-bouncy on the couch.
Remember girls vs. boys game night? Amy and Penny pinned Sheldon down and both covered him in kisses.
If we’re counting kisses that happened (or presummably happened) off-screen, Debbie Wolowitz likely kissed the man Howard caught sneaking out of the window of their house (Howard’s pediatrician, or orthodontist, IIRC).
Howard probably snogged Leslie Winkle during their time together.
Howard and Raj using the robotic mouth things.
Since they were going at it in the paintball shed and his bedroom, yeah. Leonard definitely snogged her as well.
You know, for a guy who complains that he’s a nerd who never gets the girl, Leonard has gotten a lot of girls, and they’ve all been pretty attractive.
And once Raj could talk to women, he made up for lost time pretty quickly.
Raj and Cinnamon.
Leonard kissed Amy.
Granted, it was on Blossom…
Stuart kissed Penny in an alternate reality/dream sequence where most of the other male leads are fat.
Howard kissed Penny’s friend, the “whore of Omaha”.
Raj and that nameless woman he picked up at Penny’s Halloween party waaay back in season one.
Both Leonard and Raj must have kissed Dr Plimpton at some point. :o
Howard, Bernie, Raj and Stuart must have all kissed Halley.
Amy kissed that tall English guy she went out with.
And didn’t Sheldon kiss his Meemaw?
And didn’t both Howard and Raj kiss Howard’s second cousin?
Seriously? :dubious: 
Stuart and Penny did kiss on their second date. There was no third, since she blurted out the wrong name (Leonard’s).
Leonard and some babe were snogging on his ocean cruise. We never saw it, but he was dumb enough to tell Penny about it on their way to Las Vegas to get married.
That’s a pretty non-exclusive club.
Raj and Lucy. Leonard and Leslie (assuming coitus includes a kiss).
They had a probationary kiss in her lab before getting together for real. She was underwhelmed. 