Muslim men

Do you guys know about Muslim men and their wives? 
what about the non Muslim girl that a Muslim man is having sex with on the side how does that work??
Would a Muslim man just have sex with any girl? Or does that mean something?
I need to know what I'm getting myself into
Update 
Like if a Muslim man has sex with a 18 year old non Muslim does that mean he's going to make her his wife or?