Wow. I can't believe how many guys in here simply just carry it.
For me it's not about Masculinity, it's about respect. The bag is hers, for her needs, not mine.
My wife and I have talked about this. I'd never carry her purse for her unless she needed me to for a specific reason (eg. She was hurt, or she was occupied doing something which prevented her from holding it). But as soon as she's able to again, she'll carry it.
I think it's disrespectful for the woman to ask her man to hold her purse simply because she doesn't want to.
It would be like if every time I want to play a video game, I sit my ass down on the couch and call her over and say "can you switch on the PS3, switch on the TV, set it to HDMI, adjust the volume to my liking, and pass me the controller. Thanks." - Fucking disrespectful.