In terms of protected sex the risks of stis are practically zero apart from herpes and hpv which has been said.
That's completely untrue and it's unwise to give such advice. If your statement were true, then why would the NHS GUM clinics up and down the country spend so much money on punters? - they routinely test us for a variety of STIs and offer expensive courses of hepatitis B vaccinations to all of us even though the majority of us practice safe sex. I got to the clinic every three months, I tell the nurse that I've had fully-protected sex with a couple of WGs and I'm offered a full suite of STI tests - for: chlamydia; gonorrhoea; syphilis; HIV and hepatitis.
Sorry to come over as pedantic but in my opinion, the only sensible answer to a question about STI risks that ever comes up on here, is to refer to the official advice from the NHS:
External Link/Members OnlyEven better advice is to talk directly to the staff at an NHS GUM clinic in person.
By the way I am like the OP in that I take the same precautions and I never go for OWO. At my local GUM clinic I was told that I could still catch an STI as some can be spread from skin to genital contact - e.g. if the girl plays with her pussy and then tosses you off with the same hand afterwards (with gonorrhoea being the most likely). Not trying to be a scaremonger but there is always a small risk with any kind of sex, no matter how safe and that's why we all need to be tested regularly.