I just came across this topic, and I think its a very important one.
I had the same concerns about 1.5 yrs ago. I started Avodart after 6 yrs of proscar with adequate results. Two months into avodart I noticed a huge change in sperm concentration. Ejaculate volume was the same, it just seemed way too watery. I asked the doc about it and he failed to give me an answer I was satisfied with. He told me I should be fine and if I was that concerned I should have a sperm test. Never had the test and quit the drug that day. It took close to a year but everything went back to the way it was pre-avodart.
I know this may not have been the answer you were looking for...just thought I would share my experience.
I hope you have better luck than I did. The drug just kind of scared the shit out of me.