I can only answer this from a Male's point of view, so take htis with a grain of salt.
1 - it's possible you have a latex reaction/allergy that is compounding the effects. I've found that I used to get after sex breakouts until I started using Trojan Ultrathin lubricated condoms. Haven't had one after sex with these in multiple years. Maybe having your partner experiment with different condom brands and types would help you find a solution.
2 - I also clean myself off really well immedeatly afterward. I know cuddling is what most want right after sex, but taking 2 minutes to wipe everything down with wet washcloth or better yet 5 minutes to take a quick shower has been another good practice for me.
3 - I also put baby powder on right after sex (and every morning) to keep everything down there clean and fresh and most importantly dry. Maybe not a good idea for a girl, but maybe there's something similar that could be used to help.