Hi Cycling. This is my first post here. Sorry for the delay I hope that your are still checking this thread. Anyways, I live in Japan, suffer from UC and take both Bactrim and Imuran.
Regarding Imuran, my wife and I also had the same worries. I've talked to many doctors about this and the long and short of it is that Imuran seems to be of low risk to males with regards to fertility.
Don't know about Bactrim or antibiotics affecting fertility. My doctor recommended I take Bactrim twice a week and this is simply because I'm in the process of switching from one immunosuppressent to another. Since my level of immunosuppresents is quite high the antibiotics are just to stop potential infections. Hope this helps.