Actually it is a double entendre (a phrase that intentionally has two meanings, especially if one is a bit risqué or sly). The first meaning is even if their carnal or sexual relationship does not last long. The second meaning is even if he is not a good sexual partner who does not have the stamina to make the sexual encounter last for more than a few minutes.

It means that he doesn't consider her Miss Right, but he does consider her Miss Tonight. He would have sex with her but would not have any relationship beyond that. He's not exactly the type of guy you would introduce to your sister.

I'm a big fan of this series too... And pls do read between the lines... OK? Obviously, Leonard eager to have sex with Penny, yet he does understand he's not good enough for a girl like penny.... So that's why he disguise his behavior as "just being a good neighbor" And even Sheldon can tell the difference, so he said "oh, of course" doesn't that sound a sarcasm to you two??? Then Leonard have to explain with the sentence: "That's not to say that if a carnal relationship were to develop that I wouldn't participate. However briefly." He added up the words "However briefly" after a short pause/hesitation, because he knew his desire is so obvious, and nobody would buy it that way actually, everyone knows that his words "That's not to say that if a carnal relationship were to develop that I wouldn't participate." isn't entirely the thing that happening to him.... So he added up "However briefly" to express that he's not lying, although he didn't speak out all his intention/thought... "however briefly" means however, it's briefly the thing that is happning to me....

....

I think the words"however briefly" means he didn't say out loud all his desires... the underlying words should be "a carnal relationship were to develop that I would definitely participate, and I'm very eager to establish a carnal relationship with her, and I'm not just being a good neighbor, I want more! yet I won't say it out, so briefly I'd say just so to you....blah, blah,blah,,,...."