It's not so much that you shouldn't train during those hours as how you train. It's a classic time for "still" training. Not so good for "active" ie. great for stance training or I supposed Dragon section of Sup Yin but not so great for say, a bunch of weapons or bag work.
Traditionally the very best time for kung fu training is from about an hour before sunrise to about an hour after. The qi is naturally on an upswing (from sleepiness to wakefulness) so your kind of going with the flow that way.
For the most part, I agree with everything that PM said. Not sure about the sex thing but if I had a fight coming up I would refrain for at least 2or 3 days, preferably a week. It might be a good habit all around but is it worth it? If you are just talking about a week or, heck, even a month if it was leading up to something important, then sure. DEFINATELY nothing the night before.