Is it acceptable that boyfriend invites his best female to travel with us, but I haven't met this girl once so far?

Because there are word limits in the title, I actually did not fully describe it. The issue is that I am going to Italy for a conference in May, and I asked my boyfriend if he wanted to visit Italy together, but he said no; he doesn't have many days for vacation yearly, and I agreed. But later he asked me again about the Italy trip because his best female friend wants to travel, and he told me he promised her to travel together once this year before getting into a relationship with me. He felt sorry to ditch her because he cannot go to travel with her So he proposed or his best female friend proposed that we 3 people can travel together, but i haven't met her once. I feel strange and unconformable for this kind of traveling. Also, I think best opposite-gender friends should keep distant with other people`s boyfriend and girlfriend when they are in a relationship; this is a personal boundary. I don't want to experience any drama in my relationship, but it seems he doesn't have a problem with it and said I also went to travel with male friends; yes, I did, but it was like 10 years ago and no relationship back then. And he said he doesn't mind if I go travel with my male friend. I am from Asia and he is American; he said something like a culture difference, but I doubt this is common in western countries that best opposite gender friends go travel alone even one is already in a relationship or bring opposite gender friends to travel with a couple but one of them never meets each other. I personally think it is a red flag, and he started to ask or blame me that I tried to say something to hurt him and if I still wanted to stay with him. His has female friends on social media but not guys; however, in real life, he seldomly goes for social. Could someone give me your opinion about it, culture difference or just a common thing that most people would be uncomfortable with?

Is it acceptable that boyfriend invites his best female to travel with us, but I haven't met this girl once so far?
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