Who it's for, and when
The lotus is for couples who want to slow down and find each other again, eyes locked. It's the quintessential tantric position: in that tradition, keeping both bodies upright lets sexual energy, the famous kundalini, rise all the way to the lovers' heads. You're free to see it as just one more game, or as the start of another way of attuning your bodies.
The right moment: never from cold. Rather once arousal is well under way, after kisses and caresses, or partway through lovemaking, when you feel like slowing down.
It takes a little flexibility, hence its intermediate level. Good to know: held for a very long time, the cross-legged posture isn't harmless. Long meditations in lotus have left some monks with painful sciatica, so change position as soon as anything pulls or hurts.
If you're pregnant, have a delicate back or limited mobility, ask a health professional for advice first. And if pain appears during or after sex, talk to a doctor or a sex therapist.