Having sex with anyone at any time is dangerous.
The best ways to guard against stds are proper hygiene, not brushing your teeth 15 minutes before engaging in kissing or oral, not having cuts in your mouth or on your hands, not using oil based lubes (lipstick or lip gloss on a condom break it down on a molecular level to the quality of old school sheep shin), knowing how to properly use a condom, wearing the right sized condom, and using condoms on toys then washing the toys.
That's it...the rest is all Russian roulette. Condoms break, pubes hold bacteria and viruses, people sweat while having sex, if a guy has Ed problems the condom could slip off, vaginas are naturally wet and can squirt so that can go under the base of the condom just like blood....
If you want completely safe get married, only sleep with her,use condoms, and continue getting tested together because you never know if she's out getting strange. Most women that catch men cheating or suspect don't pack up and leave, they never say a word to him then go hook up with neighbors, trainers, husbands friends.... Women are more apt to revenge hook up and hook up closer to home.So even marriage isn't safe obviously, how many of us are married? Then statistically like 70% of cheaters are being cheated on too and don't even know it.
The only safe sex is with yourself and living with like 20 cats and wearing crocs everywhere so that all temptation is avoided because once she sees your croc collection or 20 cats it's game over.