the major risk factor is blood-to-blood or fluid to blood transmission. If you've no cuts / nicks / sores on your genitals then the risk is fairly low - the skin is a pretty good barrier
Thats why taking it up the ass is so risky (for you and her) - the risk of cuts and tears is that much higher. Hence one of the reasons HIV first exploded among gays and needle users. Also one of the reasons that for a time in 1970's in the UK and USA syphilis was - almost - eradicated in the straight population. It was very much a gay disease. It came back with the increased prevalence of heterosexual anal sex. If you're not a druggie, don't go in for anal, don't bite/tear/rip skin, have a decent immune system and don't already have a pre-existing genital condition such as herpes then the risk is fairly small. Not zero, but lower than much of the hysteria would suggest.
Thing to remember is that a large proportion of the population DO have herpes, and many silently carry chlamydia - and both can be avenues of transmission for more noxious disease. For instance syphilis can be transmitted from fluid from a sore (either a chancre or herpes sore or skin tear) direct to skin - but its very very unlikely unless the recipient also has a tear.
Something to remember, and which I only found out recently is that if you think you have been exposed to HIV you should go immediately to the local STI clinic where they can give you preventative prophylactic retrovirals which greatly reduces the risk of infection. This has to be done immediately, BEFORE the diagnostics can prove the presence of the virus. Wait till then and its too late