They probably technically have enough semen to do it, but evolutionary benefit is tied to actual advantage in the actual world - not in theories. You would not evolve to do something that you have almost zero chance of ever doing. Ergo, it's a BS theory. In addition, the corollary that women are naturally choosy and sexually reticent is also BS when you look at actual nature, at primates in particular, and don't make your conclusions out of Western patriarchal norms.