It depends on your personal views. I, personally, wouldn't be able to
handle someone saying something like that to me.
If you're a jealous
person, there's no way this will work. Or a suspicious one. Do you
really want to spend your marriage with this girl wondering if she's
still in love with this man? Wondering if she's cheating on you with
him? Wondering if she married you for the wrong reasons?
Maybe these
problems would fade over time, and maybe she'll fall out of love with
this other man. But you shouldn't allow yourself to be her second
choice. In my opinion, you have three options:
A. Tell her you accept it
and propose anyway.
B. Tell her you'll wait for her until she figures
out whether you or this other man will be the better fit for her, and if
she falls out of love with this other man she can come back to you.
And
C. Tell her you're not comfortable with being put in that situation. If
she's really in love with this guy, or you, she wouldn't be saying
she's in
love with both of you. It's just not right.
I don't think you should
allow yourself to be treated like that or subject yourself to that
stress. But it all depends on your personality and the type of person
you are. Good luck!