My guess, backed up by the definitions for unisex in the Oxford English Dictionary is that this is a attempt by manufacturers and sports shops to show that they recognize that women too, hike and backpack and need tents. They are trying to overcome an image of backpacking being only for homo saps with a Y chromosome. Coming soon at a hardware store near you: unisex screwdrivers, unisex hammers and unisex saws. (Homebuilders will be in deep trouble if unisex electrical connectors come into fashion.)
From the OED, unisex:
A 3. adj Designed to be suitable for either sex; not peculiar to one sex.
B 3. n 3. Also with capital initial. The state or condition of being sexually neutral or indeterminate; the quality of being equally suitable for both sexes.
The OED gives several examples, of which the most pertinent is:
1969 Sunday Mail (Brisbane) 11 May 24/8 It's unisex where men and women have abandoned the old ‘vive la difference’ school of thought in dressing.
Substitute recreation for dressing, and you have the "logic", however weird, behind unisex tents.
Still in question, who started this idiocy about unisex tents, and where?