My boyfriend talks sexual over text with his friend and she slaps his thighs is this normal?

I am very jealous that this girl talks sexually with my boyfriend and slaps his thighs when in school. I've brought it up to him and he is very confused on why it makes me uncomfortable I've tried to explain it to him three times now. He is a very smart guy and is a good boyfriend but gosh this makes me jealous and this girl has a girlfriend. Should I be jealous is this something to be worried about? I am scared tbh my jealously is off the roof with this. He and I got in a little tiff over it. I'm thinking of hiding this jealousy considering I've brought it up to him three times now. I feel bad because this is their friendship and he told me that if he changes it will cause them to have to change their whole relationship. I don't know what to do, please any advice you have.
Updates
+1 y
Hello! We have completely moved on from all of this he understands my boundaries and we respect each other. Our relationship is now a year and 3 months in. We are completely healthy and now respect each other whenever we feel uncomfortable about a situation or a friendship! Thank you everyone for the advice!
My boyfriend talks sexual over text with his friend and she slaps his thighs is this normal?
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