Happymommy, if you're thinking what I think your thinking of what I'm thinking, then, no, I haven't seen any problem with it, and have never felt it was a sin.
I have also found no indication in the Bible that it is. There is more to physical intimacy with one's spouse, besides just the main course.
I did LOTS of research on an example of "seed spilling" aka Onan on my blog. My Domestic Church: Search results for Onan. Feel free to read those over.