Talk:Taiyang Xiao Long/@comment-24.206.70.32-20151024190711/@comment-4010415-20151024224258

An easy explanation is that Taiyang and Summer didn't get married. A lot of women have their child take her last name if she births the child out of wedlock.

Taiyang likely married Raven, but then when Raven left, she pretty much decided "I'm never, ever coming back, so consider this a divorce. Please move on with your life and be happy" and went back to her maiden name of Branwen. It's implied that Taiyang may know more than he told Yang, considering Yang said "He wouldn't tell me everything."

Anyway, I pretty much headcanon and theorize that Summer became pregnant after a moment of weakness on Taiyang's behalf turned into a night of passion. She was simply a good old close friend (for all we know, they could've dated before Taiyang and Raven got together), who came to his side in his hour of need and lived with him to help him take care of and raise Yang. Behaving as parents together, they may have begun developing feelings for each other.