Why do men check out other women ??

Why do men check out other women wether discrete or not when they are with their wife or girlfriend ?? What do you ladies think they are thinking about these women they check out ??

My husband checks out other women discrete he won’t turn all the way around to check another women out but if their happens to be a attractive women in front I’m sure he looks but I don’t usually say anything just bc i feel its normal right !!! Unless they are turning heads then that’s just a no no !!