Is it okay to feel really awkward by bf's mother kissing him on the lips goodnight/goodbye?

Okay so I'm concerned because normally i see mothers kissing their son/daughter goodnight or goodbye on the cheek not the lips, not to mention my bf is older than me (17) and i don't know if this is normal or not. I love him to bits, but it makes me feel embarrassed and I shy away from him before he kisses me because I'm afraid I'll get his mother's saliva in my mouth somehow. She doesn't like, full on kiss obviously because that's weird, it's just a brief peck that lasts for a second. I don't ever intend to be horrible to him when i move away, and I've explained to him why i do and he was fine with that. I mean.. I dont know it just makes me feel uncomfortable.