People feel sleepy after orgasm for several reasons.
- Your breathing goes from rapid, shallow pants to slow, deep breaths.
- The physical release is very relaxing. There are all kinds of hormones released during orgasm. Seratonin is one of these and gives a person a 'high.'
- Sex happens most often at night, when your body is winding down anyway.
- Sex usually occurs on a bed so you are lying down when it is over. It's only natural to feel more sleepy and relaxed on a bed.
As for the urge to urinate:
The contractions and spasms you have during arousal and orgasm press against the bladder, causing the urge to urinate.

......And try to urinate...to see what happens!