Veganism is, for most, a matter of ethics above all else. To put it simply, semen resulting from sex between consenting adults could be considered "ethically sourced" in the sense that it is produced and "supplied" freely/willingly, not taken from another by force or through domestication. So, from an ethical stand point, yes, it's "vegan". From a nitpicky technical one, it's not because it is still an "animal byproduct". It's really up to the individual to decide which standard they choose to follow.
This is not terribly complicated. And I'm not even vegan.
