My parents split up when I was around six years old, eventually getting divorced later on. My dad did not want to divorce, but my mum did - basically my dad was young and (regrettably) did not pull his weight as a father although he has always been an amazing parent to my in living memory (I am 25). My mother also wasn't willing to deal with her issues so it wasn't going to work.
Since splitting up, my parents have had other relationships but never remarried. They never had any further children. When I was 17, my parents got back together for a brief period, no longer than around a year. They have remained 'friends' but I find the closeness difficult to explain to other people and most of the time don't bother trying.
Right now neither are in a relationship with other people and they spend a decent portion of their free time with each other. For example, I was recently surprised to visit my sick grandfather (my dad's father) and find my mother already there helping with doing his washing - like a wife! 
On top of this they have been on going on what I consider to be dates and now my dad has asked to meet my boyfriend at a ffamily friends event...with my mother in tow. My aunt always thought they would get back together but I am not sure if this is what is happening and don't feel comfortable with raising the issue with them. If they are getting back together, I have mixed feelings on this too.
I really want to go to the event but my boyfried knows my parents are separated so don't know how to explain they are attending together. I find it a bit awkward and embarrassing and worry how it will appear to a) him and b) what his family will think. Help?