It is. =|
I don't believe in too many "man rules" but the urinal one is definitely one I follow.
With regards to "off-limits" girls - I don't read too much into that, it depends on the circumstances. I know a lot of guys who were happy enough for their mate to start dating their sister (don't know how I'd feel about it...) and a few that have gone out with the ex of a mate. I know in my case, I've done it, although in one case I hadn't seen the guy around for months and in the other case I asked the friend first to make sure he'd be alright with it. But yeah, the heart goes where it goes, etc.
Current girlfriends, though, of pretty much any decent guy will be off-limits for me. I don't agree with the idea of breaking up a relationship unless it's a clearly bad relationship, and even then I don't make any move until I know they've called it off.