Kissing her gay best friend on the lips?

So I am dating a very independent girl and she is really open-minded. We can talk about nearly everything and she will be okay with it.
Recently, she asked me is it okay for her to kiss her gay best friend on the lip because that is what she used to do when she was still single while they are partying together. I know what the guy looks like and can confirm that he is gay. However, even though he is gay, it doesn't make it right. I feel really uncomfortable about this and told her that "I know he is your best friend, kissing him in the past is fine because you were single. I am okay with you guys holding hands or hugs but kissing is where I draw the line and as your boyfriend, I feel really uncomfortable about this". so she replied, "he is gay, nothing is going to happen between us". and I said "Yes, I know he is gay but why do you feel the need to kiss him. you have a boyfriend now and it makes me uncomfortable seeing you kissing other people and it's not just me I am sure most guys will not be ok seeing his girlfriend kissing another person". I know some of you may say that there is no harm in kissing a gay friend but for me, I am not ok with it because a kiss should be something special between us two.
Anyway, after that conversation... she told me that she would stop kissing her gay best friend if it making me uncomfortable and she doesn't want me to be upset. I'm sure her gay best friend will understand because she is now in a relationship. I know she is open-minded and she always tells me things but what if she goes out partying with him and still kissing him without telling me. Then I will never know right. I know I have to put my trust in her and believe that she is telling me the truth. But I still find it hard to not worry and believe that she won't do it again because like I said if she doesn't tell me then I will never know.