multiple default route base on physical interfaces

for incoming traffic, you do not need route (e.g. default route) to send return traffic because bigip uses auto lasthop to send return traffic to incoming mac address.

sol13876: Overview of the Auto Last Hop setting (11.x)

for outgoing traffic (e.g. pool server initiates traffic to internet), generally, route is required. anyway, you can use performance l4 virtual server with gateway pool instead of forwarding ip virtual server with route.

LTM: Per-VLAN Default Gateways by Deb Allen