Returning to the OP – been in a similar situation, halfway through a degree wasn't the best time to be starting a family!
Went through a whole range of emotions and options, but toughed it out and came out the other side. Have two kids and no regrets now.
Accept that your lifestyle MUST change, and plan ahead. Lots of good stuff on here about the money side of it. One thing becoming a dad taught me was how much time and money we had wasted before. When there isn't much of either, every pound and second becomes precious and more worthwhile.
And I only began to ride after becoming a Dad – I gave up the gym membership as we didn't think we could afford the time and fees – since then been through 6 complete bikes and two frames in 6 years. Oooops!
It's the easiest thing in the world to walk away when faced with this colossal life changing event, always harder to tough it out.
Looking forward to your vasectomy thread when you decide to take no more chances!