"I slept with Dylan FIRST"
It really seems to me that Dylan is NOT the father of Kelly's son... Remember Jennie and Shannen have already said that their characters won't be fighting over men. If Dylan were the father, Brenda (and all of us) would feel quite ill-at-ease...
I couldn't stand the love triangle either, the biggest mistake ever done in 90210. It was like the beginning of the end: a divided audience (Kelly vs Brenda fans) and the show really became a soap.
That's why I think the early seasons (1 & 2) are the best, the more realistic, and the ones everybody remember, from China to Patagonia.