I Feel Like A Tool!

So let me explain my mother and I a little bit.  No good story is built without first seeing the foundation.  My father died when I was 7.  My mum has been in about three serious relationships in the 17 years since his death.  She has casually dated, had a few minor relationships.  But in general she has lived a solitary life(well, minus her one beautiful child....oh...and those other three).
A while ago she started 'seeing' this man named Rob.  About a year and a half now I'd say.  It started as a 'casual' relationship.  He lives in a town a ways away and his job requires travelling.  So they met up when he was in town, went out that sort of jazz.  Then the became exclusive.  But it didn't last long.  He was pushy, sexually from what my mom has said, he demanded she do things she wasn't comfortable with, he would go weeks without calling her then show up on her doorstep and demand sex.  This may seem like more than I should know, but as we get older, the female children have become more friends then kids with my mother, she doesn't really have friends as she works from 5am til 10pm five days a week, and 8 hour shifts another day.  And this man was demanding she spend her available time satisfying his whims.
I've never met him.  My big brother has never met him, my little sister has never met him.  Only my older sister met him, and that was by accident.  Never before, in any of her three previous relationships, has she kept him apart from us for this long.
So she calls me today and asks me what my plans are for the next few Mondays.  I tell her not much why?  She says she wants to arrange a supper so her kids can meet her new fiancee.
My stupid head response?
"Did Rob ask you to marry him???  Did you say yes???  Oh Mom...."
Instantly I could feel the hurt coming through the phone.  And I felt like crying a nano second after I said it.  But just two days ago she was telling me about how Rob is against her being baptized, and how he thinks her religion is a crock and he tells her that he won't support her.  In order to marry him, after she is baptized, he NEEDS to do certain things for her marriage to be accepted by the church.
So we talked for a few more minutes.  I followed it with a lame-o "Congrats on your engagment Mom".
She told me in those few minutes that she has no ring because he is 'saving' to get her one.  He isn't in town, so he must of proposed over the phone.  I honestly haven't met the guy, but alot of red flags have said that he isn't faithful.
I'm really upset.  I have nothing against my mum getting remarried.  It's been 17 years for crying out loud.  She deserves to be happy.  But I think she is doing this because she doesn't want to be alone.  I don't think she is going to be happy at all.
And now I have to go to an engagment dinner and act like I like the guy.  Or that I approve of this.