Three of the 4 male leads have seen Penny naked. The exception, obviously, is Howard. (I am assuming that Penny actually disrobed during her curtailed romp with Raj).
Not to mention spanking Amy…
When I said “coming close” was acceptable, I did not mean “something somewhat vaguely sexual”. Sheldon touching Penny’s boob while helping her dress-- not coming close to having sex. Leonard changing his mind at the last moment because he didn’t want to cheat on Prya-- coming close to having sex.
And, since I am declaring myself judge of what coming close is, Sheldon and Amy’s D&D tryst is not coming close because they were not close to having sex. Sure, they were as close to a sexual act as Sheldon seems capable of, but that doesn’t mean they were close to having sex. Quite clearly, sex was not gonna happen.
And the spanking… ?
Didn’t Howard hook up with a hometown friend of Penny’s who to stay at her apartment?
You underestimate him. I have complete faith that Howard saw Penny naked long before any of the others did.
I don’t think he was in the same room or even the same building when it happened, though.
Yes, “The Slut of Omaha.” Penny was not a fan of the coupling or the girl.
This is who I meant by Penny’s slutty friend in the OP.
Didn’t Howard gift Penny a Teddy Bear with a camera in it once? Yeah, could have seen her naked I suppose.
Played by the preternaturally perky Brooke D’Orsay from Royal Pains.
I believe Penny’s slutty friend was the Whore of Omaha. Howard’s mother told her to “go back to Babylon you whore!” and Raj said, “I thought she was from Omaha.” Classic comedy gold!
Actually, “The Whore of Omaha”. 
IIRC, she was more than just aroused. When she touched his hand in the bar, she visibly shivered and then said something like, “Well, that should hold me over for a while”, implying that she achieved spontaneous orgasm.
And after finding the link, I had it basically right.
What was the episode that Howard and Raj hooked up with a girl from Comic-Con?
It’s mentioned in 5.22 “The Stag Convergence,” but I don’t think we ever actually saw it.
Nobody has mentioned Howard and the robot’s arm? One of the funniest episodes ever.
Different episode but…
“Unless my people were freed by Benjamin Franklin and his five twin brothers you are wasting your time.”
Masturbation isn’t usually considered sex. But if we’re counting it- Raj wanking in the same room as Priya when they were growing up should go on the list.
Well, if a semi-incestuous Teens for Jesus hoedown didn’t count, I don’t think that would either.
Well, there was Richard The Slave Girl from Comic-Con.
http://wiki.the-big-bang-theory.com/wiki/Richard_the_Slave_Girl