Ik this sounds bad but there is a lot of context behind this my (former) best friend(20F) and I(20F) have been friends for around 8 years at this point and are currently in our our 3rd year of university but at separate universities. I met my now boyfriend(20M) about 2 and a half years ago and the 3 of us all got on really well at the start. Me and my boyfriend broke up a couple months in to our relationship but got back together about 6 months later and have been together for 2 years since.

A few days ago my best friend and I got in to an argument after a while of back and forth between us my friend started saying how she was confused why my boyfriend invited her to his 21st birthday party, I explain he was trying to be nice extending an olive branch as she's my friend (she had previously complained in the argument that we hadn't been talking and I hadn't visited her at uni yet this semester), so we thought it would be nice to invite her, she went on to say she didn't want him to extend an olive branch, she didn't want him to invite her because, as she kept reiterating, she does like him, she doesn't want to talk to him and doesn't want to be around him. (She has told me previous months ago this was because of an argument she had with my boyfriend on a night out that myself and my boyfriend thought had been resolved as we had all hung out together since multiple times and everything was fine).

I said to her it is fine if she doesn't want to go or be around him but she doesn't have to be rude she could just say no thanks. She then went on to send me a huge paragraph explaining that for "years" my boyfriend has made he feel uncomfortable, sexualised and has inappropriately touched her. She said 'years' So I assumed this all started pretty soon after we met (we both met my boyfriend through a mutual friend on the same day). She carried on explaining she felt that I always her brushed off when she brought an issue about my boyfriend to me, however, she has never come to me with anything to do with my boyfriend before and I have always listened to her. However she later said she had voiced these concerns to me about my boyfriend sexualising her, touching her and making her uncomfortable. This conversation never happened though and I don't know where any of this has come from.

She then also told he sexually assaulted her a few months ago and said she had previously told me about the sexual assault and claimed I didn't believe her and dismissed her. THIS NEVER HAPPENED. She went on to say i was supposed to be her best friend and i "can't even do that" and called me a bad friend over a reaction i didn't have. I'm so confused. we've been friends for so long, and I don't know why this is happening.

I, of course, spoke to my boyfriend about this, and he obviously deneyed ever doing any of this. My boyfriend was molested and also abused as a child and through out our relationship he has been very clear that he despises anyone who commit such crimes like sexual assault, rape, abuse and the general disrespect of another human being, which makes me think he wouldn't have done this as its not in his character.

My friend ended up giving me an ultimatum of her or my boyfriend, and I'm torn. I love my best friend and we have been through a lot together, but she's lied to me in the past, but never on this large of a scale. She has also wanted me and my boyfriend to break up since we got back together 2 years ago.

When I originally told her we were getting back together she was not happy told me how we shouldn't, not because of anything my boyfriend had done but because of me, she told me I would just hurt him and lead him on only to break up with him again and I shouldn't put him through that. Clearly, she was wrong as we were still together.

She would also tell me to break up with him every opportunity she got when ever we had a disagreement or bickered and I would just vent to her and say "yeh he annoyed me a bit today', she would automatically tell me to break up with him every time. She has also lied to me before and told me my boyfriend tried to hit her during an argument they had when I was in the bathroom at a bar when out with some friends. I later asked around the friends we were with and the bar staff, and they all confirmed during the argument that they never saw him try and hit her or show any aggressive or intimating behaviour towards her. I later brushed this situation off as they had seemingly made up and and I thought there may have just been a miscommunication as we were all a bit drink.

I'm also very confused as to why she is uncomfortable by my boyfriend, as just to name a few examples of her behaviour, she has previously gotten changed in front of him before completely stripped down to only her underwear without any warning, he was respectful and looked away, to which she proceeds to say she doesn't care if he looks. She also jokingly gave him a lap dance (that only lasted a few seconds) without any warning in a bar, he looked away and kept his hands by his sides, she has also talked about wanting a threesome with me and my boyfriend which we thought she was joking about at first (she wasn't) and it was quickly shut down after, and has also invited my boyfriend to feel her ass to prove a point to him. So I'm very confused as she instigated all these situations and my boyfriend was very respectful in all of them.

For context, my best friend is a lesbian and I am very secure in my relationship, Ik my boyfriend would not turn his head especially not for my best friend and as she is a lesbian I have previously had no reason to be concerned about her behaviour for the most part, it has always been in a joking and lighthearted manner.

So I'm completely torn, but I'm leaning towards believing my boyfriend more, but I feel wrong for not believing my best friend.