On one hand, Mike Lynch hates kids. He can’t stand how they scream all the time, poop themselves, everything. That probably comes from his father reminding Mike throughout his childhood that he was never wanted. On the other hand, that ice of indifference surrounding Mike's biological clock starts to thaw after his girlfriend gives birth to someone else’s child.
Mike believes everything has a reason for existing, including our naughty bits. He now has a desperate urge to fulfill his basic human duty by using those bits to procreate. He just doesn’t want to deal with the aftermath. To get around this self-imposed dilemma, Mike takes a three-day weekend trip to Indiana, finds some women who are also looking to get laid, and tries to get them pregnant. He justifies his actions by assuming the people he encounters are going to get pregnant during one-night stands anyway. At least with his being the sperm donor, their child will be born to serve a greater purpose: Mike’s.
When he returns to Indiana a year later to see if the scheme was a success, confronting the results of his actions elicits more emotions than he knew he had. Now Mike has no idea how he’ll react when he discovers his child.