It is ultimately up to you and what you feel comfortable with. But since you asked...
I tell people. When I was in the rapid weight loss phase it was something I couldn't hide (heck, didn't WANT to hide) and I won't lie about it. I am proud of the decision I made to get my life in order and fix my health problems. I dieted and exercised for YEARS without success, so it obviously was not just diet and exercise (though I truthfully do watch what I eat very closely and exercise regularly) that got the 160 pounds off. It is too much work to make up stories and keep straight who knows and who doesn't.
Now, does that mean I wear a sign around my neck announcing to the world that I had gastruc bypass? No. But if someone asks how I lost the weight or something related to it comes up in conversation, I'll talk about it because it is part of who I am.
Example - Last Monday in my Teaching Methods course we had to give "personal history" presentations with pictures. For the first time since my surgery I labored over the decision to tell my classmates and professor about my weight loss surgery because I had the exact same fear as you do - these people have never seen me at 300 pounds. They do not know that I used to be so big, and they never have to know. I don't want them to disrespect me because I took what many consider "the easy way out". But then I remembered - I am not ashamed. Weight loss surgery is NOT A DIRTY LITTLE SECRET. It isn't. Star Jones makes me SICK because she outright LIED about it, making it seem like it is something to be ashamed of when it is something you should be proud of. Magazine covers that go out of their way to say "look at these people who lost lots of weight WITHOUT surgery!" - as if losing weight with surgery is something you should be less proud of are needlessly putting down surgery as a method of achieving weight loss (I mean kudos to people who can get 100+ pounds off and keep it off without surgery - good for them - but that doesn't mean weight loss achieved with surgical intervention is any less remarkable or earned than theirs). And ya know, if someone would choose to not like me or value me less because I used to be heavy, then to hell with them because I don't want people in my life who wouldn't have valued me as a heavy person. I was a worthwhile individual when I was big and I am proud of the person I have become as a direct result of having lived as a morbidly obese individual. I hated myself then, but I can appreciate the surviving and overcoming I had to do the further out I get from surgery. So I told my classmates in my presentation because you cannot talk about the person I am without talking about who I was, and who I was happens to be a morbidly obese woman who needed a way out, so she found one with RNY. And the response was unanimously positive.
I understand why you would want to keep it a secret. I really do. And no one is saying you need to tell anyone you had surgery and used to be MO on a first or second date (though it might be kind of obvious depending on if you decide to go out for a meal or not...). But I want you to know that WLS is not something to be ashamed of and having been obese isn't either. With time I hope you come to be accepting of the person you were before surgery and that person deserves respect from you and anyone you might choose to let into your life romantically.
Amy 293/140 - AT GOAL!