Once guys get out of their teens/early 20s, ladies definitely don't expect them to be ready the moment the underwear comes off unless they're ladies who don't have much experience with adult men.

As guys mature, it take more time and more foreplay to get them ready. That's just a fact of life. Women love the foreplay time themselves, and then when we're applying our magic touch (whatever, however) to guys to help get them hard, that's a very empowering feeling. We like to know we had some influence in causing that reaction, you know? At least I know I do. Hope that makes sense. So, again, no, seriously, we don't expect guys to be hard the second their underwear comes off.