When they start holding hands, get worried, other than that I guess there just friends, and, as far as I can see there is nothing wrong with that.
Guys don't look at each other like competition the way women do.
My wife's answer to this is if they don't like it, then don't look, and I say if she doesn't like being looked at then don't do it.
As fro complaints then yeah it happens, but less so if you good looking, if your but ugly and overweight then expect the complaints.