Vegans do not consume meat or any by-products, since they believe it is healthy or that they protect the animals this way. It is true, but in case of semen, no such criteria is fulfilled. Semen is full of vitamins and minerals and no living thing is harmed by swallowing it – unless a woman considers it as a kind of abortion – so refusing to swallow it is illogical from this point of view.
It should not be considered as any kind of dairy or meat by-product, since it only contains protein, which might be found in meat, but our clothes made of cotton also contain hair of an animal, and we still wear them.
However, one must understand that vegans refuse to eat eggs – regardless of whether they are fertilized or not, thus they do not even consider that not every egg would give life to a chick – and from this point of view, swallowing semen might conflict with their principles.
Yet, swallowing semen does not involve any cruelty – unless they consider it as cruelty against themselves – so if a vegan uses her diet as an excuse, then you should rather talk about her inner motives, doubts and fears, it might lead you closer to the solution. Also, if you also follow her diet on the day before sex, then your semen might even taste better, thus if she does try it, she might like it and would be willing to repeat it later.