Hi lotsofchoc, just wanted to give you an alternative viewpoint.
My dh's brother has a genetic condition with means he has a zero sperm count. He and sil are desperate to have kids. They are soon eligible for IVF and were asked to consider whether they'd like to use a donor known to them or an anonymous one. They then asked my dh if he would consider it. Bil has another 3 brothers he could have asked but chose to ask dh.
I didn't know dh had been approached initially. Dh pretty much decided he'd like to help them, so he asked me if I'd be ok with the idea. I'll try to be as honest as I can about my thoughts.
Initially, I was overwhelmed with the 'jealousy' thoughts I had. I thought (and still do to some degree) that I'd be the only women to bear dh's children and I couldn't get my head around another women doing so, and the thought that my dh was so willing to allow this. It just seemed/seems so much to go against our exclusive relationship. Stepping aside from these major issues, I couldn't help but think about other practicalities. Eg, one of my ds looks very very similar to his cousin - they could be brothers. But obviously they are not, their dads are brothers. But what if bil/sil's baby (with dh's sperm) had a strong resemblence to my dc. (Btw, bil/sil have not told anyone else in the family about the struggle ttc, ivf and possible sperm donation). There would be all the 'oh doesn't he look like ds, they could be brothers' etc. It happens a lot now with just cousins, so I know it could happen with sperm donated half brothers. And how would we all cope with these comments. Another one is, we have 3 ds and have probably completed our family. I have no idea how I would feel if the donated sperm led to a girl. Or for that matter how dh would feel. It would be the girl he never had. Another major issue is that kids now are allowed to have information regarding donated sperm/eggs. So there would be an obligation to tell the child when older that s/he was borne through donation. And if s/he wanted they could find out that it was their uncle who 'fathered' them. I cannot envisage how this would affect the dynamics of the whole family, including gparents, my dc etc. In our situation bil/sil still do not want the family to know anything. I can't get my head around how we can all do the transition from nobody knowing to everyone (especially the child) knowing.
I am very aware I am looking at our situation from a very selfish stance. Dh on the other hand is being selfless and aultraistic. He tries to understand how I feel. And I think we know nothing would happen unless we were both completely happy. Dh told bil we need some more time to decide (I was pregnant at the time). Although dh is selfless, he's not very proactive and has never spoken again to bil about it. I think they are both waiting for each other to broach the subject. Meanwhile I had a heart to heart to bil. He told me that they also asked sil's sister's dh at the same time. He apparently has said yes. So the dilemma has been taken from our hands.
My first thought when I heard this? It was relief tbh. Swiftly followed by thoughts of 'so what's wrong with my dh's sperm then?' and 'my dh's sperm will be far superior and lead to more beautiful children'. In fact I felt a teeny bit indignant. This made me think some more about it, and ultimately I probably would have supported dh in helping his brother. But I probably would have wanted openness about it all within the family.
Sorry to be so rambling. Just wanted to give you an insight into being on the other side. Ultimately I think what you and my dh were/are prepared to do is genorosity in the absolute extreme and to be applauded. (I just feel uncomfortable with the 'my dh is fathering somebody else's child' thing.) Running away now, cos I know I'm a selfish cow.