I think you misunderstand what these drugs do, they do NOT increase libido (the desire for sex). They are PDE5 inhibitors and allow for the blood vessels in the penis to dilate by inhibiting the affects of PDE5 (see the technical explanation below)
Explanation (lifted from www.medscape.com/viewarticle/542736_4
Following sexual stimulation, penile erection occurs through the release of nitric oxide (NO), which causes dilation of the blood vessels of the corpus cavernosum via an accumulation of cyclic guanosine monophosphate (cGMP). PDE5, an enzyme that breaks down cGMP, can be inhibited and thereby the vasodilatory effect of NO is enhanced (Figure 1). All three PDE5 inhibitors share this common mechanism of action and are only pharmacologically active when cGMP synthesis is activated. The action of the PDE5 inhibitors therefore requires sexual arousal.