Fallon, your response was perfect. Here are some suggestions.
Do start keeping a log. Every ding-dangity time she says you haven't met the right man yet, write it down with date, time, place, and your response. Keep one of those little notebooks with you. If Sandy is a good friend, get her in on
all of this, and ask her to keep a log, too.
You could even do it in front of her. When she asks about it, you can wryly tell her that you think she doesn't realize how often she tells you to leave your husband.
Now for RooRoo's scripts.
1: a toned down version of David H's suggestion. Ask her, directly and seriously, "Are you really telling me to leave my husband?" If she responds with anything resembling "No," then say something like "Then don't. say. that. again." Use an assertive tone of voice.
1a. At this point (and/or later), she'll probably accuse you of being too sensitive.
You, in a serious but friendly tone: Well, how would
you like it, if, every time I saw you, I said, 'You shouldn't have children! Nobody should! The child-free life is marvelous! Give your baby up for adoption!'
Cow*: "Bla bla but I wasn't saying..."
You: "You telling me I haven't found the right man yet is the same thing. You're telling me to leave my husband."
2: Cow: "Oh, you just haven't met the right man."
You: Raise eyebrows, silently look her in the eye for a few seconds... "Isn't (local sports team) doing well/badly this season!" (Or any other subject that has nothing at all to do with families or children.)
3. Raise your eyebrows, look her in the eye for a few seconds... Turn away. Ask or tell Sandy (or anyone but Cow!) about something you have in common. You can escalate this further by being even more obvious. "How about that local sports team?" Without naming any team, just saying "local sports team."
4. If, after a few doses of this, she still doesn't get it, it's time for the Cut Direct. Instead of following the Absolute Silence Look with beandip, you follow it by turning your back on her. If you're at your desk, ostentatiously (but silently) get back to work. Afterwards, talk to her
only if your work requires it, with no smiles.
If even this does not work, then it's time to take your logs and go above her head. You have done everything you can, short of throwing a screaming fit. She is creating a hostile working environment for you, based on your lifestyle choices. It might be hard for them to take it seriously, especially if they have children. There is always somebody that rides their hobby horse too hard. But telling you you haven't found the right man, when you're married?

Be sure to point out that she is
telling you to leave your husband.
*

It's just an abbreviation for "Coworker." Honest!
