Not in general, though there is the case of Cloud, formerly of the Defenders.
An amnesiac, sentient nebula who'd taken imprints from two humans when she crashed on Earth, her assumed form was of a naked human female (naked apart from some discreet clouding up of appropriate body parts for a CCA comic) but on occasion "she" would switch and take on a basically similar but decidedly male form. Whilst at first it might have been meant to suggest that she was mimicking a human form and every now and again just got the XX/XY genes mixed up when constructing a solid form, eventually it was revealed that she just happened to imprint on two young lovebirds who just happened to look almost identical to each other. IIRC she ended up returning to her own part of the galaxy, but kept a human imprint around because she'd enjoyed the experience.