taking a valtrex prior to sex isn't going to help at all. it takes at least 5 days of taking it daily to reduce the risk to a partner.
if you are a female with hsv2 and have a hsv2 negative male partner, if you did nothing but avoid sex any time during any genital symptoms, he is 96% likely each year NOT to contract hsv2 from you. If you also use condoms and take daily suppressive therapy with valtrex, he has on average a 99% odd each year of NOT contracting hsv2 from you.
does daily suppressive therapy lead to resistance to the antivirals over time? no it does not. the rates of antiviral resistance are about the same even after 25+ years of medication being available.
http://www.medhelp.org/user_journals/show/8280/how-should-I-treat-my-genital-herpes is a journal post I wrote a few years ago I think you would benefit from reading too :)
keep asking questions!
grace