Chairs, like most furniture, can be an endless source of erotic amusement. Sitting on a chair is especially good if you're pregnant. While he's sitting, you can straddle him, facing away or towards him.
There's good eye contact if you face one another. The man is in the ideal position to hold his penis and rub it against his partner's clitoris or stimulate it with his fingers. The woman's hands are also free to caress his testicles or to grab his buttocks and pull him towards her.
Sitting positions increase a woman's tightness because of the sharper angle for entry.
The woman can use her legs for leverage to maintain a steady rocking rhythm as he thrusts.