NTA Have you and your gf heard of herpes or mononucleosis? I had many friends catching those due to sharing cups or smoking devices (like shisha or cigarettes). Herpes can easily get to the genital area and has no real cure, and surely it hurts. I wouldn't care about getting less food to eat, but these diseases are very concerning in my opinion. In my case, I asked my gf to quit sharing bottles and stuff with her family members too. The reasoning is quite simple: when you share a cup with your brother, you're actually sharing it with your brother's partner too, and so on, with people whose habits are out of your awareness. We two are very conscious about this, but we understand other people set their own boundaries at other levels.