Derek is, at least as far as Patrick knows, straight, while Maya is gay. We don't know how Patrick would treat Derek if he also was gay. As far as i understand, Patrick objects to Maya's orientation mainly because it means his wife/their mother won't see come true her lifetime wish of witnessing their children "get married and have children of their own". A wish he's made a cornerstone for his daily struggle, the one thing/reason for him to continue his own efforts.The fact that he totally treats Derek differently from Maya is inexcusable. This is what really makes me think ill of him.
I'd say it's unhealthy and totally unfair to Maya, but i can also see where he's coming from here in what's effectively became his obsession.