There Are Two Factors At Work Here. 

There are two factors at work here. 

1) She doesn’t see your penis as a penis anymore; it’s been in panties all day!

2) She doesn’t see you as a man anymore.  What kind of self-respecting man wears panties on the order of another?

When this whole cuckolding thing started, you were still her equal.  She had her fun with Brad, and you still got plenty of intimate time with her.  But now that Brad has ordered you into panties, you’ve become the beta in the relationship.