I thought it was worth an update as this post generated a thought-provoking amount of response for me 18 months ago.
A lot of the negative comments were valid, based on my personal (& selfish) feelings at the time. However, I always try to do the right thing and despite these initial feelings would have "stepped up" in the end.
Whilst my eldest son, who is now 18 (& at University) has little contact with his son, I have developed a strong relationship with both my grandson and his mother (daughter-in-law doesn't seem to fit). She has already had a second child with another man, but relies on my support with my grandson to share the burden, both emotionally and financially. Whilst her parents are less supportive since she became pregnant again by this other man.
My second son, who is sixteen, has moved back in with me and we are closer than ever, which is nice and I still have a strong relationship with my younger son and daughter (who still live with their mother).
It's also cleared things up for me regarding future relationships. Whilst my ex-girlfriend wanted kids of her own and I went along with it, even though I didn't. I am now very clear with any future partner that I want no more kids and am honest about my time having moved on to supporting a grandchild. This has made me far more relaxed when entering any new relationship.
Often at weekends it'll be me with my second son and grandson (the nephew of my second son) doing stuff and having fun, which is a slightly odd family make-up but enjoyable none-the-less. The only major sadness I have is that my oldest son (the father of my grandchild) visits rarely and is very distant.