Took the words out of my mouth, Woodsman's Game
The best any of us can do is write the way we learned from childhood, and try not to laugh too hard when we come across the language barrier in other stories. As far as "knickers" go, the only time I've ever heard that word spoken aloud was when I was watching "Are You Being Served" on PBS, and the was Miss Brahms. Based on that and that alone, I took it as sort of a commoner, lower-class word. I know better now, but it still has that connotation for me. As for the American alternative, panties, for us that denotes lingerie without saying lingerie. Underwear is my tighty-whities. My panties are made of satin. If I'm writing erotica, panties it is.
There are no hard and fast rules for what words we can or can't use, and a multilingual site like this is bound to have a few things that make us chuckle. "Arse" always throws me off--there's nothing sexual about it in my mind. To me, it always seems kind of archaic. As far as the alternatives for "penis", "engorged manhood" is just so overused, it's funny. Every bad romance novel flies back in my face. But to me, "enormous fuck-tool" and the like are porn, not erotica. Each to his own, I suppose. I can get past most of it. Spelling, grammar, and verb-tense switching are the things that make me click the back button.
Morrighan