If you engage in any oral fluid exchange (kissing, licking, oral sex) then there will be small microscopic souvenirs floating around in your mouth for some time. Your body, and your skin, have natural defenses, so this isn't a huge deal usually.

If you have cuts in your skin, or direct pathways from the outside to the inside, then any of your souvenirs could get directly introduced into your bloodstream, reducing your natural immunity defenses. Unnecessarily.

Brushing your teeth, and flossing your teeth, can cause abrasions and bleeding, even microscopic un-noticeable bleeding, which is enough for a pathogen to transverse the blood barrier.

So to be safe, don't brush your teeth, don't floss your teeth, immediately before or after sex