It's 2020..Is their a polite way to tell a man he should go down on his girlfriend more often than he does or he should learn to try ??

Nobody wants you. You cant get women, you cant get men either, go ahead bend over all you want your going to get rejected. Get a job or a hobby 

ask, open the lines of communication, tell him what turns you on, be a teacher and show him and yet be teachable, maybe he doesn't know that you like that also do you reciprocate and give him the same pleasure.

she should go down on him to show him

Fill it with strawberries and whipped cream then cuff his hands behind his back and tell him dessert is ready!

Vaginas are full of bacteria. Gross and smelly and juicy

Some people do not like fish.  BTW, will you go down on him, many girls will not do a guy.

Just do it