I would probably be offended but Id try to play it off like I am not. I know girls get off on damaging a guys ego/self esteem so I wouldn't want to give her the satisfaction of knowing she got to me. Even if it was someone I didn't give a shit about it would still hurt. Its kind of an ego thing to be good in bed, but it also gives you power. Girls often complain that most guys suck in bed, so if you're one of the few that is good she will try harder to keep you. So its kind of something to strive for, to be one of the few good ones.
Also if a girl spreads rumors that you suck it will damage your reputation and make it harder to get dates in the future. Then conversely if she says good things then you might even get referral sex from other girls. Its not that common but I have heard of it happening where girls have wanted to sleep with a guy just because she heard from other girls that he was good.
Oh another reason why its beneficial to be good in bed is if you ever dump a girl because of her past, they won't be able to use that cliched "oh you're just insecure because you suck in bed" line. If you're undeniably good in bed then they've got nothing to throw at you.
A - If that were to ever happen then I would be extremely offended (and I don't get offended easily)
I've been told this by more then one girl/girlfriend, was tough to hear and upset me every time but never have really gotten feedback from them as to why I wasn't good in bed when I tried to be. Girlfriend broke up with me over it. Makes me nervous to approach girls for dating and sex now
Wow, there are a lot more virgins than I thought. Im also a virgin so all that is yet to come. If it was my first time and the guy said "your shit in bed BTW" I would say "fuck off. . . its my first time, you can expect it to be absolutely perfect". A guy with no patience at all. I wouldn't see how a relationship like that could last long. But hey, if you already have the knowledge. Its not rocket science so there shouldn't be much going wrong.