My family thinks my bf will f my cousin

Hiii, as the title says my family for some reason thinks my bf will try get with my cousin! I never understood it they’re good friends and all but he’s never hit on her or anything

She’s good looking and nice but they’re just friends. What do people think of this? Is it in a man to do it even if he says they’re just friends and he likes her as a friend..?

I have never had any issues with my BF trying to get with other women even though we are semi open