“Believe whatever you want” trust me, when a woman says this to you, another man is pipping her. When a woman doesn’t care if you think she’s cheating, it’s because she’s cheating and isn’t scared of losing you.
“If you don’t do it for me, another man will” this is the most obvious indication that your girl is sleeping with someone else. It is like a subtle threat to let you know if she has other men in her life and you should be on your best behaviour