I hope this information helps:
There are a lot of factors dealing with erections, sex drive, etc... Aside for testosterone, 5AR, estrogen, SHBG, DHT, (2-Carboxyethyl)arginine synthase, etc, it's been noted by some that dopamine agonists have worked better for sex drive than 'test-boosters' (for lack of better words).
Some natural products would include Powerful, L-Arginine + Pycnogenol, L-Phenylalanine (or DL phenylalanine)....
The dopamine re-uptake inhibitor buproprion (aka Wellbutrin) is known to greatly increase libido and erection frequency and intensity in men and increase libido and orgasm length and intensity in women (and men). Dopamine is the hormone responsible for orgasm and the increase in prolactin after ejaculation is a result of diminished dopamine levels there-after you climax.
Certain substances that inhibit dopamine channels such as GHB are responsible for delayed onset ejaculation (curing premature ejaculation).