INFURIATING. I’m so sorry you heard them talking about you. It’s one thing to privately disapprove of a co-workers personal life (even though it’s STILL nobody’s business but yours), but they really crossed a line when they make it the subject of office gossip…
Unfortunately, you can’t “make” anyone understand your perspective, because people like this just refuse to understand (especially anyone who would vocalize their judgment of your situation… what nasty little $*@$s. I’m sorry if you’re friendly with any of these people, which I imagine you are, but this kind of thing really ticks me off). In the end, you’re absolutely right: it’s none of their d--- business what decisions you and your boyfriend make about YOUR individual lives, and your life together.
And another thing -- how presumptuous of them to assume that it’s your boyfriend who “won’t commit.” Yeah, because everyone knows, a woman’s goal is to get that ring as fast as possible! Oh, brother.
Are you particularly close with any of the people you heard gossiping? If so, maybe you could take him/her aside (OUTSIDE of work) and tell him/her that while you didn’t mean to eavesdrop, you accidentally overheard their conversation and it really hurt your feelings to hear a friend say such things about you and your relationship (not to mention how very unprofessional their behavior was… might leave that point out).
Then again, confronting them might only exacerbate the problem. I suppose it will really depend on your relationship with these people, and how you think they might react. I would be mortified, embarrassed and emphatically apologetic… and I’d vow to keep my gossiping trap shut forevermore.
Above all, maintain your own professional behavior at all times. This is a workplace, after all.
Again, I’m so sorry you had to deal with that. Try not to let their insensitive gossip get to you. What matters is that you and your boyfriend are making the right choices for yourselves, and not according to what anyone else thinks is right for you.