I noticed the same thing with my work khakis. I had Haggar khakis. I think they were classic fit 'work to weekend' Haggars. I like them but they were just snug down there.
I saw this thread, read it, but didn't find anything that I wanted to try from the suggestions.
Well, just yesterday I was doing push ups at work. Ive been doing 80 push ups per day and spreading them throughout the day 20 at a time. I get done with my 20, I bend my left leg to stand up, and the crotch rips.
So I finish up a few emails, pack up my tablet, and head out for a work travel trip. Before I leave for the trip I go straight to the mall to replace my ripped crotch pants. I didn't have a ton of time so I just wore the ripped khakis in the mall. The tear was pretty well underneath and I had dark underwear on so you couldn't see anything scandalous, but you could see the rip from behind.
I ended up getting Haggar'a again but this time I got the 'Cool 18' khakis. They are designed for golfers, they are light, and they are stretchy and flexible. It's almost like a microfiber material. I actually bought grey, not khaki, but they do make them in khaki.
So, I don't know if they actually have more ballroom. They feel like they do compared to my Haggar's that ripped but it might just be the material. I did the catcher sit and they felt great. I feel like they have a little bit more room compared to others, and the material is stretchy and flexible so they are comfortable in the crotch.
Anyway... that is my recommendation. Try the Haggar Cool 18. I had the same problem you mentioned and so far they feel like a big improvement.
