I don't think it makes you a sinner. Sin is such a meaningless concept, if you ask me. It's strange that this lady called you a sinner for this. I don't recall anything in the Bible speaking of such a thing. For you see, the Bible and the concept of sin deals less with prohibiting what harms others, and more to do with what allegedly offends God.
ANYWAY, where I was going with the above is that while I do not think it is a "sin", I think that people should be more respectful of others. In your case, the other stalls were taken, so I kind of understand it... But it needs to be said, some people don't have the option. If you take a restroom that is the only option for them, while you had other options, then it is rude.
I have a transgender friend who has been treated like dirt at her job. They don't allow her to use the women's restrooms. They force her to use the Family restroom. But there is a problem there. Now you see, it gets worse than that. She is (or was? I don't know anymore lol) a cashier, and cashiers can not freely go to the bathroom for obvious reasons, they can only go at designated times. So, you know, when someone else is using the Family restroom, she doesn't have a bathroom to use, period, and could have to wait hours on end, or not get to go at all if enough people were using it when she needed it. I kid you not, she told me that the manager who told her that (even though the company's policies are less strict) kept on using the family restroom, right in front of her. Along with everyone else, even though it denied her the right to have any bathroom to use whatsoever.
It frustrates me. Family restrooms were never created so random people could use the bathroom privately - they were created so single parents with children of the opposite sex would have a bathroom they could safely take them in. I know that initial purpose doesn't include transgender people like her, but I think it's only fair that it would include them (when they run into an issue like this manager) and people who don't pass either way.
People should be more respectful of others... If you have two options for a bathroom to use, and you know someone else doesn't, don't take the option away from them! It may seem minor, but as in the example I gave above, it isn't always that way.