Brett, you have nothing to worry about. The only thing that defines one as gay/homosexual is a sexual attraction to the same sex. You have clearly stated, and you know yourself best, that you are NOT gay. That is all that matters.
Wearing a thong, having a manicure, wearing your hair a certain way...these are all "fluff", they do not indicate one as "gay". What they do, however, is symbolize gender roles (a man should act/talk/dress a certain way, and females also do in a certain way), and I find these to be culturally defined.
Example, in Asia, some cultures are permissive of men holding hands with other men. There is no "malice", no one thinks the men are "sleeping together", it's considered "normal". In prudish societies like the USA (I have to say it, America seems so Victorian in its' view of masculine and feminine behavior), even glancing to see another man can get you beaten up, under the suspicion that you are "cruising" another man.
Your example is a case in point. If you wear something that is culturally defined in America as "something females normally do", you will be accused/thought of as gay. Even though it is totally untrue, you will still be thought of as such, as you so rudely found out.
And you are correct in assessing that people who think this way are not worthy of your time. The fact that they would laugh and call you names suggest that they really do not have anything going on in their lives. I would stay away from people with too much idle time - though I would suggest that they get a career in gossip magazines.
Enjoy your thong - for you. =)