Well, I'm not sure if my response will be much use, but me and my twin brother are actually the result of a sperm donation. My parents had been trying to have children for years, and had even tried the donation 6 times previous but to no avail, until finally they got two at once. It was the most amazing gift anyone could have given them, and they do not regret it for a second.
This could mean I'm biased in my opinions, because obviously without the kindness of sperm donation I would never have 'come' about (sorry couldn't resist). Therefore, I think if I found out my boyfriend had donated sperm, I would have a lot of admiration for him as I know the joy that can bring to an empty family, and it's a huge thing for someone to do, especially as nowadays children are allowed to track down the biological father.
To answer your second question, as a donor child, I do think about half siblings quite a lot as I know that most likely, I have up to 10 of all different ages and it's a very strange but also somehow comforting thought. If I was a donor, I am sure I would think of it too - but you have to remember that what you did was a gift of kindness to parents as opposed to a gift of parenthood yourself, so really I feel your first child with your partner is still the first time you become a parent as I feel that a parent is the person who is there from day one and loves the child and cares for them. I think it's important to get the difference between parenthood and donation, although this is just my opinion.
I think honestly I would feel more comfortable if my boyfriend was not in contact with his donor child, because I think it's just healthier that way. The child has been born into a family that love them that much that they are willing to go through the process of expensive fertility treatment in order to have them, and I think it is important that they recognise them as their mum and dad. As a donor child myself, I do not feel any desire to know my donor because as far as I am concerned, the man I call Dad is my father, biological or not, because that is the role he has played since I was born and that is what defines him as a father. Of course I do have curiosities, but even if I could meet him (I was born before the law was passed allowing me to meet him) I don't think I would because it just doesn't matter. All I need to know is that he was kind enough to give the gift of life to a couple who couldn't do it themselves, and that's satisfying. I think it's a bit of shame personally that the law has been passed where children can contact the donors, as this means there are a lot less of them, but I understand arguments for it - again I'm a little biased from my own situation.
I hope this helps! Sorry it was a bit waffly...