Hmm this situation is a little difficult for me to feel about. Honestly Karin seems to be quite promiscuous as she seems to kiss anyone (girls so far) who asks and given what I could tell asks to kiss others she sees. I do like that Mei recognizes she has also kissed a few other girls.
However, there’s a big difference between Mei and Karin. Where Mei is looking for love and a girlfriend and is confused about her feelings and seemingly falls in love fast and has mostly been the recipient of attention, Karin seems to just want to kiss and has more of a history of playing around.
This is my observation and opinion of what I‘ve seen so far.
I will say that personally I find it proper etiquette to wait for a reply after asking to date exclusively, or whatever being girlfriends means to them, before being intimate with another. So I feel Mei is justified in feeling Karin may be playing around a bit. (Again good of her to notice her own transgressions at this point)
I just hope Mei can see through the cloud of lust and have an honest conversation before proceeding with anything.

