Huh. I never get that response when people learn I speak Portugese. Two possible reasons:
1) It never takes long for them to figure out or learn that I was there as a missionary, and if missionary was busy learning swear words, they'd rather not know.
2) It's French specifically (the language of love) that they get a kick out of knowing swear words for. It seems that you could be saying the most romantic things to somebody in German, and it would sound like you are cussing them out, but you could be cussing them out in French, and it sounds like you're saying something cool. As many issues as I have with French culture (in many ways, it has become what I am most afraid American culture will some day become) and the French language (It seems to be a language specifically engineered to make sure that those d**n foreigners from learning it properly*), I still have to admit that French sounds way cool when spoken.
* I'm mostly kidding about that, but it seems like that sometimes. Why is French pronounced so differently from the other languages that use our alphabet? Why does it seem that over half of all letters in the language are silent?
Concerning 1), there were some missionaries who went out of their way to learn all the swear words. I was never one of them. But about halfway through my mission, my companion made sure that he taught them all to me. Not with junior high type motivations (tee hee! these words are naughty!), but because he felt that it was important to be able to know what people are saying.
It turned out to be a really good thing that I knew some of the cruder words later in my mission.
Sometimes, especially when playing board games, I do "swear" in Portugese, but never using actual Portugese swear words. I just say things like "son of a mother" or "how rotten". It's not really swearing, but it's nice to have an explitive that everybody at the table doesn't automatically know isn't really an explitive (like darn and fetchin' heck!).