We groom ourselves all the time to be more physically attractive. Whether it be hair, muscle, or whatever. I think to an extreme degree, it sends an unconscious message, which is to say that you aren't even aware that you are sending it. That message is that you are still available. In fact, going to great lengths to ensure that your hunt for a mate is successful and that your availability is obvious to everyone who looks. Men will look at you and size you up to determine what threat you may be to them and women look at you to consider the possibility that maybe you are a worthwhile mate. All this happens in just a few seconds or less and is not even perceived by your conscious mind.
But nevertheless, most people, men or women, can tell you when sitting in a restaurant or club, who the most attractive people are and where they are. You probably never even noticed the other person scoping the room... but they did.
Anyway, what I'm getting at is if you are showing all the signs of a male working to attract mates or a better mate, then of course your current mate is going to be sensitive about it. They may not even know why and say something like, "Well your with me now so that just doesn't seem cool."
As far as what BN said, don't be too hard on these guys. Most of my best friends are loyal and faithful friends and they are definitely not GQ, MH, or Flex material. A girl is with them because they are kind and are probably attracted to them because they feel they are safe because the guy shows no outward signs of looking for someone else or someone better. That's all.
Add to that, that many BBers are self-centered, vain, insecure little dickheads, that anything other than as a trophy are worthless. That doesn't mean all of us because look at all the cool guys right here on this board. Some of them would give you the shirt off their back! Even more so if you allowed them to bust a few poses after they gave it to you...