I think the description of Guy A is lacking. While it sounds very noble and romantic, "I'd give up everything for my love" is a really boring character trait, and not actually a basis for a relationship of equals and partners. If he's a really giving and loving person in general, it'd be nice to know that in other ways, so he comes off rounded and loving instead of desperate. Also, like, whats fun about him? What does the MC love about him? What does he love about MC?
Guy B I think it really depends on the kind of person MC is. For some people, a partner flirting with others would really be crossing a line. Especially if she is a more insecure person, this would be really hurtful, and it'd damage trust between them. If she's more confident she might either not put up with it, or she much understand it's just play and trust him. It really depends on the dynamic.