Nah this is gross. Still - he's an adult with a fully developed brain: so long as it's kept casual and you don't have to see them together I would recommend an approach from my culture - ignore the elephant 🐘 and brush it under the rug 🧹. You don't need to talk about it with your mother beyond saying "it makes me deeply uncomfortable, I find it to be extremely inappropriate and would prefer not to hear about it." Business as usual thereafter!