Platonic relationship & responsibility of the offspring

A man donates his sperms and a woman conceives an offspring using it ... Later the father is exposed to the grown up offspring. Should the father be forced legally to take the offspring's responsibility?

I think he should not. If the lady really needed somebody else to share the future resposibility (which she must be aware of this while artificially inseminating herself) she shouldn't have done that using an unknown's genes in the first place.

What u guys say? The question is inevitable becoz its happening in many places around the world....now-a-days