Sewing Tip: When sewing a fly zipper application, it may be helpful to only topstitch the crotch seam partially at the top, starting at the desired zipper end and stitching about 1.5"- 2" down. Once the fly is fully stitched and all zipper components attached, the crotch seam can be completed all the way down to the end of curved seam.
When the crotch seam housing the fly is only partially stitched, the fabric lays flat when marking the placket line and attaching the zipper and its components. When sewing a fly-front zipper application, you want to be able to move the two seam edges and garment portions apart as well as lay them as flat as possible on the table during the marking and zipper alignment process. The more comfort and flexibility, the more accuracy you'll have in the construction process, especially when machine stitching.
Another aspect to consider that some areas that need to be ironed during the zipper application process will be easier to handle, without the added tension and pulling otherwise created by a fully stitched curved crotch seam. The smoother and flatter all garment portions lay, the more comfortable it will be to complete the zipper application process.
Keeping the body of the garment as flat and tension-free as possible will also help you topstitch the zipper placket as evenly and smoothly as possible. This mainly due to the fact that if the bottom portion of the crotch seam is left open, the top portion will lay complete flat which will naturally, provide a more comfortable canvas to accurately topstitch on.