Personally while I would have loved to see it be ANYONE but Conner get Cordy pregnant, it would have been very difficult to write the season with a different sire for Jasmine, mainly because it was Cordy's manipulation of Conner that made it so easy for her to hide what was happening. Although I really felt it was a poorly written storyline, much of it did hang on her concealing who she really was and what she really was up to by emotionally manipulating Conner (who was very immature and not used to interpersonal relationships). Still, that being said, Groo was too to a degree....and much less icky. I guess it would be a big leap of faith though to think Groo was THAT clueless (especially when it really got down to it). I don't think Groo would have stood by while Cordelia ordered him to sacrifice a virgin in order to jumpstart Jasmine's birth.
Crappy season though