I don't think there is a prescribed amount of how often we females require sex. I personally feel as if I could go without sex for the rest of my life (although once I get going I usually change my mind!) and I guess everyone has different sex drives. I don't know what effect Viagra has - maybe you could ask your doctor for more adivce? It is probably best to talk to your girlfriend about it - say that it's nothing personal, but you feel your libido is low for *whatever reason*. Many people just go through phases where they have lower libidos than they usually do, or higher libidos than they usually do. Maybe you could suggest some alternative fun which might take the pressure off a bit - having some fun without actual intercourse.