The next time she brings it up, say "I'm not interested in dating anyone else while I'm still married to you, and I don't believe in open marriages -- PERIOD. Now, please don't bring it up again."
Nip that.
I believe she is either currently, or was while you were gone, dating other man/men. It would be very much "script" for her to suggest you do the same, to relieve her guilt and to create a once-again-level moral playing field.
