Yes. There're some situations in which a girl has liked another
guy for years, but because he wasn't available then, or due to
whatever other reasons, she didn't end up with him.
After she had gotten involved with another guy, somehow the guy
she's liked all along came back to indicate interest, & she
risks her current relationship with her bf to go out with the guy
she's had an interest all along.
It's hard to determine the reasons for people doing this kinda
thing to their partners. Suffice to say that this situation occurs,
& will continue to occur till the end of time.
FeowFeow