Is it normal my brother denies cheating on his gf?

My brother keeps going out at 9pm with different women each night. He claims they are his friends and that his GF knows this. I ask why he sleeps over their houses...he doesn't answer that. My Mother believes him and doesn't mind it. My Father thinks he's going to end up hurting his GF. He only sees his GF twice a week yet he sees a different woman every night. I told him to just stop dating his GF if he won't even tell her where he's going/lies to her but he says he isn't cheating. So I keep pushing, asking WHY he sleeps over and he just keeps telling me he hangs out and ends up just sleeping over/that it's normal and they sleep in separate rooms. Either my brother is gay or he's a cheater, right? My brother is 20. IIN?