i (18f) have a boyfriend (17m) of about 4 months, and his birthday is coming up soon. he has an ex girlfriend he dated for a few months in late 2023. they are still somewhat friends and talk, although not regularly, she recently suggested to him and some other mutual friends (we are all part of the same friend group) that they should have a joint birthday party, as their birthdays are 12 days apart. he seems open to the idea for the sake of convenience, but as his current girlfriend, i really don't like the idea of him throwing a joint party with/ for his ex's birthday. is it unreasonable to be upset at this? what should i do?
tl;dr
my bf wants to have a joint bday party with his ex and i don’t know how to feel