Thanks for engaging on it.
There's a much larger conversation going on, about the role of gender in technology. Two of the core topics are 1) the presence (or lack) of women in computer science / web development, and 2) the respect given to the women who are present.
Even if it was unintended, directing a question on an open board to "gents" suggests one of two things: 1) there are only men here; 2) even if there are women here, I'm not interested in their insights. The first is false (HN does have women, and would be improved with more). The second is disrespectful and would also lead to a poorer results set.
I'm sure there was no disrespect intended on your part. Indeed, I'd believe you were trying to make the question more respectful.
Either "How do you make blueprints for your web application idea?" or "How should I make blueprints for my web application idea?" would carry the exact same intended payload, while removing the gendered language issues.
As a follow-up to my earlier point, I'd really recommend the 37 Signals blog and the writings from Amy Hoy. Just about anything from their back catalog will be helpful as you map out your ideas, and as you make sure the product you build is actually something people will want.