Somewhere in the game there's a warning about this. The frequency of proposals will drop as time goes on. I can't remember where it is now, and I don't have the game installed on this computer so I can't check.
I think the most suitors I've ever rejected is three; I panicked a little and accepted the fourth without vetting him carefully first. My peep still managed to have three children with him. I don't know if proposals ever stop completely, but would imagine they become rarer and rarer. If you haven't already, it might be a good idea to buy her a cat.
