Who it's for, and when
Missionary suits almost every body type, as long as the erection is good and firm. Good to know: a height gap between partners doesn't matter here. It's a comfortable basic position that opens the door to a whole host of fun variations.
It's the position for couples who love looking at each other and kissing while they make love. The one underneath can let go completely and think only of her own pleasure: that's her absolute right, and she's only passive if she chooses to be. The one on top picks the rhythm and depth, and enjoys the view of her breasts.
The long version is made for evenings when you've got time to spare. Alternate it with quicker sessions: served too often, even the finest dish ends up losing its appeal.
Precautions: the position gets trickier when one of you is very heavily built, and a heavy man can crush his partner (hence the forearms, or the kneeling posture). After the first few months of pregnancy, it's best avoided: ask a health professional for advice, and think about side-lying positions such as spooning. If penetration hurts or your mobility is limited (raised legs call for a little flexibility), talk to a doctor or a sex therapist.