Honestly if the guy really was my bro I'd be like smash that shit dude it's awesome. The only way I'd ever be mad is if I was hoping to get back together with the girl, otherwise I'd be cool with it. Of course, I'd also expect the same attitude if I wanted to fuck his ex.
If they had a kid that would be a little bit different IMO, but in general I'd say better someone I know than some douchebag.
Alright, so that's a general no, with the exception of blazedglory. A friend of mine from work - let's call her S - is in this kind of situation, and asked for my advice. Apparently this guy (J) and her friend (K), who moved away a few months ago, were really serious. S said that while they were dating, she considered J a friend. They've been hanging out recently though, and she's realizing that she has really strong feelings for him. (S and J went out on a few dates pre-K&J dating, but at the time she wasn't that into him). This whole situation got me thinking. As a rule, I say don't. I also understand where blazedglory is coming from, but I think it takes a very mature group of people to be able to handle things playing out that way. It's interesting to get other people's perspectives. I try to be as non-judgmental as possible in this position, so that a friend can talk it out openly. I told her that I can see things from both sides, and if she really thinks that she might love J, then to listen to that. But to be prepared for the fallout, i.e., hurting K and possibly ruining an important friendship. I talked to S the other day, and she decided that for now, she's basically cutting off contact with J.
No issues with this as long as you mention it to me first. I just see it as a common courtesy. Of course kids with said person would complicate this