It would bother me, but I probably wouldn't say anything about it unless it was them spending loads of time alone constantly.

I don't really agree that sitting on a couch watching a movie is going to lead to anything. Well ok I suppose i can see the girl cuddling up to the guy, but they're not going to start having hardcore super sex. I think the problem is that I (and other guys) view girls as being a sort of helpless being and that if she's with another guy and he starts flirting that she is going to just forget about her relationship and suck him off. In reality, if she cares for the relationship she won't do anything like that - no matter how tempted. The exception is, of course, when alcohol is involved. But even then, the girl should have enough sense to know she shouldn't get so wasted she'll do something stupid.