I can fully understand your insecurity about such a thing bro I have gynecomastia so pretty much the opposite lol. not a nice feeling either way but I don't see the point of lying about it. If you are just looking for a one night quickie then by all means do and say what you need to in order to get your end wet, you could even tailor a cool story how you were doing a double backflip on your dirtbike and got impaled on the handlebars or some other manly man stuff like that. but if you are looking for a LTR then don't tell a lie about something like that.
Pretty much all in all do what ever makes you happy. but it is something that is part of you and I don't see why you should lie about something you can't help. and if the chick can't get over it then NEXT that B!TCH into oblivion