Maybe talk to her and ask if she was safe in giving them each a blow job. If she says no, then maybe suggest going and getting tested for STD's. I mean in all honesty it's none of your business what she does to other guys and whether or not she has/hasn't contracted something from one of them. It's her lifestyle and her choice. She chose to do that and not be smart about it. Yet, if you still want to try and do something just casually talk to her and see if maybe you can beat around the bush on getting a test done. Hopefully she's smart and doesn't have anything, but it's going to be her choice in the end on going and having a check on STD's done.
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I love you!