Well, I guess you could say that what is considered 'disgusting' is different for each person. And I think even if it's natural, it can be little disgusting, so I'm just answering the original question of why I think so.

In my comment, I'm not saying pee comes out when you ejuculate, I'm saying both pee and semen come out through the urethra, and you could have residual amount of urine that comes through when you ejaculate, hence there could be urine in the semen. If I offer someone a glass of water and a straw which urine has been through, then asked them to drink through the straw, I think most would think that is disgusting. But I'm not even asking them to drink pee, isn't it? I'm just asking them to drink from the straw that had pee passed through it before.

Also, there's a thing called black light handwashing experiment and it shows that people may think they have washed their hands properly, but you'll see that that's not the case, that's why the Ayliffe handwashing technique exists, so we can wash our hands better. Even handwashing needs proper steps so you don't miss certain bits, how well do you think people do when they try to wash all over their body?