TBF I think it's usually the guy in a couple that asks to try this sort of threesome, not the woman. Being one of 2 or more men having sex with the same woman is a pretty common fantasy amongst men, even those that consider themselves straight. If a woman wants a threesome, she'll usually want the 'special guest' to be a woman.
If she asked to try a threesome of either sort I'd be open to the idea, but I'd want to talk over the 'why' with her first.
If she wants to broaden her sexual horizons and experience new things that aren't possible with just one guy, then cool, I'm up for it.
If, on the other hand, she is using the 'threesome' as a sort of compromise to get me to agree to let her fuck other people, or a way to fuck other people she can rationalise as 'not cheating' then nah, and we probably need to be having a bigger conversation...