I believe that a Sheldon & Penny relationship gets fondly called a paradox because the two characters are absolute opposites of each other. According to Wikipedia, the definition of a Paradox is: a paradox is a logical statement or group of statements that lead to a contradiction or a situation which (if true) defies logic or reason.

I think it's considered that Sheldon would be one statement and Penny would be another statement that tied together would be a contradiction. They are so different, almost polar opposites, that putting them together would definitely almost defy logic. At the same time, the paradox of a Sheldon and Penny pairing is probably what people find so intriguing and amusing about the idea of them as a couple. So I think that calling them a Paradox coupling is both accurate and done with fondness.

As to why talk of Sheldon and Penny as a should be kept to their shipper thread in TBBT forum... I don't know if it's fair to say it absolutely has to kept there. However, a Sheldon and Penny romantic relationship is not what is happening on the actual show. It's an opinion (and I'm not trying to say it's an invalid one) of some people who feel that the two have chemistry and really, really want to see them together romantically. For others who see them as a more brother/sister pairing, there's something rather disturbing in thinking of the two of them together romantically, like it would almost be incestuous or something. So there are some people who see them together romantically, and there are some who look at that suggestion and feel downright sickened by it.

Which is why, for the sake of not fighting each other - because we all love the show, and we have that in common, and I think that we're all nice people and want to get along - we try to keep some of the romantic shipping of certain couples in their appropriate threads.

It might not seem fair to talk about Sheldon and Amy but not be able to talk about Sheldon and Penny, but the thing is... according to the show at this time, Sheldon and Amy are a couple and the plot of the show is actively developing their relationship. Their relationship is actually happening on the show we are watching. So to talk about their relationship is to talk about the reality of the show. To talk about Sheldon and Penny in a romantic relationship... well? I don't know. Is that actually happening on the show we're watching, or is that wishful thinking of people who adore the chemistry between them and want to see it turn romantic?