Yeah, a tip: grow a pair of nuts.
I know you’re a decent guy because you cared to ask me and there isn’t a lot of support for dudes in your position. But you have to do it, dude. You’re going to be worrying about this for the rest of your life with any girlfriend you have if you don’t just get it off your fucking chest. Might I mention that life gets a lot harder than giving a shit if your parents think your girlfriend’s hot?
Start with your coworkers. Worst case scenario: if they tease you about it, try to keep it light, make fun of their ugly girlfriends back, keep it cool and equal. If they’re continual assholes, talk to your supervisor about sexual harassment. I’m not kidding. I wouldn’t put up with that shit where I have to work everyday. I should abate your fears though. It’s probably all in your head, so have some faith in humanity: most people have met a fat person before and know basic etiquette. And if someone’s an asshole to her face, by all means defend her honor. She’s your fucking girlfriend and you’re already lucky she hasn’t dumped your ass for not introducing her to your friends and family.
Your family? You’re under no obligation to tell them your sexual preferences, so bring her home and let them bring it up. If and when they do, keep it brief: “I think she’s hot and it’s not up for debate. End of story.” If they go “health, blah blah blah” tell them it’s her business what she does with her body. Strongly suggest they drop the subject and if they don’t, leave. You have to force people to deal with it on your terms. Why? Because it’s who you are and you can’t live life worrying about being rejected because the majority of the planet has no idea how rude and stupid and shitty they are about this one fucking thing. Repeat after me: It’s not my problem, it’s theirs.
And last but not least, you should come out because you could be rewarded with free porn for making a deserving effort on behalf of all the cute fat girls who’d rather not date a coward who lets other people stand in the way of his happiness.
Read this. Then take a deep breath and get it over with. You’ll thank me, and more importantly, she’ll thank you.