We all have our limits, yours are obviously this side of interracial sex and GBs. Perhaps she lied because she knew that and didn't want you to judge her (as you are doing) based on experiences in her past. Whether or not you told her that there's no way you would have moved forward had you known those factors, my guess is that you've talked about enough things for her to have an idea of what things you might consider taboo.
That said, I'm not saying she lied. I don't know her.
What I do know is that she has some friends who seriously lack discretion and since they lack discretion it's possible that also stretch the truth or create stories where they don't know facts. We've had many instances where stories were created about things we had done (that never happened). People see us with certain other people and assume things are going on that are not. When people are drunk their memories can not be relied on; whether it's because whatever they may have seen or heard while drunk got muddled, or because later when they are drunk they muddle things, merging stories they heard with things they've seen elsewhere.
There is only one person giving you a version of the truth here that you know well enough to take at their word and that is your GF. The question is do you trust her enough to take her at her word and let her past be her past? Or don't you? If you don't, things will only get worse from here.
That said, you've already said that you doubt you see swinging in your future, and based on the fact that she still has a lot of friends in the LS, I do see swinging in hers. You might want to discuss that with her and weight it in your decision as to whether or not this relationship is worth continuing with.