AITA for not immediately comforting my gf?

I (19m) just got into an argument with my gf (20f). Basically, my cousin (15f) is coming into town in a couple months and wants me to take her to look at colleges. My gf said she doesn't want me to take her just the 2 of us in a car alone. I asked why and she said because she doesn't trust my cousin not to try something, and she forbade me from taking her alone. Instead of comforting her, I asked why she doesn't trust me around my cousin, and also mentioned that in addition to being my cousin (and genetic half sibling) shes underage and I would never cheat on my gf in the first place. Her reasoning is that my cousin is still a female, and that my paternal cousin (21f) did something weird to me (she put her head on my shoulder when I was 16 during an early morning Latin church service). I again expressed I was upset that she doesn't trust me and apparently thinks im a perv, which caused her to shut her phone down for over an hour and then after I got her on the phone she got mad I didn't immedialty comfort her. AITA?