I've tried these ones and have not personally experienced any side effects...
Wild Oats (Avena sativa)(arial part)
Epimedium (Horny goat weed)
Red Panax ginseng
Deer Velvet Antler (Lu Rong)
Herba Cistanches (chinese herb)
Extracts, especially dry extracts, would be more likely to upset stomache. Some extracts must be taken with food, but it depends on the person. I've observed much variability in stomach sensitivity to high-phenolic type extracts, like green tea or rhodiola.
As to Orion's comments, totally agree with the other factors he mentions. As to the comment on efficacy of my recommendations, I make the assumption (maybe I should not) that increasing general libido and "yang" type energy would increase the semen volume overall, at the end of the day type of thing, cuz your ejaculating more overall. Anyways....can we talk about something else? Before Bob Dole chimes in.