I often find my handbag is incredibly heavy, but its rare for the bulky/weighty items in it to belong to me. The 3 guys in my family (husband and two sons) tend to offload things into my bag because they don't carry one on most family excursion (eg: to the shops)
If my shoulder or back was sore, or the bag got too heavy (happens when they sneaked books that weigh a ton), I'd have no reservations asking one of them to carry my bag. If they complained about giving me a hand with it, they'd regret it.
A handbag is not a piece of furniture or a box of groceries, it's part of a woman's outfit. If you wouldn't wear your girlfriend's coat or shoes, then you shouldn't be carrying her bag around.
Note I did say "if", I don't want to exclude anybody
Carrying around different to standing obviously holding it for a couple of mins while your partner goes to the bathroom!